Part One of Six.
Here is another answer to the question about how long a planet - a planet with a radius of 2,142 kilometers, mass of 6.594 X 10 the 23rd power kilograms or 0.1104084 the mass of Earth, and thus a surface gravity of 0.98 g and an escape velocity of 6.41 kilometers or 3.983 miles per second - could retain its atmosphere.
In my previous answer I wrote that if the oxygen in the exosphere of the planet's atmosphere, where gases escape into space, had a temperature of 1000 degees K or less, and a root-mean-square speed of 1.25 kilometers per second or slower, the planet could hold an oxygen atmosphere for about a hundred million years if it had an escape velocity of at least 6.25 kilometers per second.
Since your planet has an escape velocity of 6.41 kilometers per second, it may be able to hold on to an oxygen atmosphere for a hundred million years.
My previous answer also said that with the radius and mass specified in the question, the planet has an overall density of 16.018949 grams per cubic centimeter, which is a lot denser than most naturally occuring elements. Your planet would have to be composed almost entirely out of one of the densest known elements, which would be rather unlikely to happen naturally.
So perhaps I should suggest an alternate method of enabling a small planet with a radius of only 2,142 kilometers to retain its artifically produced atmosphere for long periods of time, without giving that planet extra mass requiring an improbable density.
Part Two: Percival Lowell's Main Error.
What was Percival Lowell's big error in his theory of the Martian canals?
Lowell believed that the planet Mars was slowly losing its water. Molecules of water vapor in the upper atmosphere would have been broken up by ultra violent ultraviolet rays into atoms of hydrogen and oxygen. The ultra light hygrogen would move too fast compared to the escape velocity of Mars and would escape from the planet, never again to combine with oxygen to make more water.
This process happens in Earth's atmosphere, and probably was a main cause of the actual lack of water on Venus and Mars.
So Lowell believed the hypothetical Martians would make the best of it by using their dwindling water supplies efficiently, distributing snow melt water from the polar caps all over the planet with a system of canals, delaying their inevitable doom.
And apparently Lowell never thought that a better method for the Martians would be to keep the water from being lost from Mars by stopping the breaking up of water molecules and the escape of hydrogen from Mars.
How could the Martians do that?
How would humans living in hypothetical Moon bases prevent their air and water from escaping into outer space? By building totally enclosed Moon bases where air and water could not get out and would be endlessly recycled.
So the hypothetical Martians could have solved the problem of Mars slowly losing water by building larger and larger totally self contained "Moon bases" on Mars, perhaps eventually totally covering all of Mars with an airtight roof to prevent air, escpecially the hydrogen necessary for water, from escaping.
Astronomers in Lowell's time didn't have very good views of Mars. There was no way they could have been able to tell if they were seeing the actual natural surface of Mars or a planet wide roof made of more or less transparent or opaque material covering all of Mars and holding the atmosphere in.
Part Three: The Roof of the World.
So possibly the humans in your story decide to terraform a small planet with a radius of 2,142 kilometers which doesn't have the extreme density you ask for. 2,142 kilometers is a little less than the radius of Callisto (2,410 kilometers), which has an escape velocity of 2.440 kilometers per second. But Callisto has a very low density compared to Earth. Mercury is a bit larger (2,439 kilometers) but has a density much closer to Earth's density and has an escape velocity of 4.25 kilometers per second.
So it might be possible for your humans to find a planet with a radius of 2,142 kilometers which is massive and dense enough to have an escape velocity of about 4.00 kilometers per second, without the planet having a core made out of gold or some other rare element. And that might not be a high enough escape velocity to retain the artificial oxygen rich atmosphere they plan to produce when they terraform that planet for long enough.
So they could build a roof over the entire planet, with giant airlocks to allow spaceships to land. And fill the space below the roof with the artifical atmosphere they produce. That would be a massive project, but terraforming a planet is a massive project.
Part Four: A Roof of Nano Machines.
I remember once looking at, but not reading, a science fiction novel by Arthur C. Clarke and a collaborator. That might be the one that mentioned life forms in space which evolve to attack and consume parts of spacecraft. Anyway, in that story the Moon had been terraformed and given a breathable atmosphere. To prevent the atmosphere from escaping into space a sort of a roof made of gazillions of nano macheines was built over the Moon. Each of the tiny nano machines was linked to its neighbors, and the spaces between were smaller than molecules. So air particles which hit the "roof" were bounced backed instead of escaping into space.
Part Five: Planetary Roofs Supported by Air Pressure.
According ot Wikipedia's list of theoretical megastructures:
Shellworlds or paraterraforming are inflated shells holding high pressure air around an otherwise airless world to create a breathable atmosphere. The pressure of the contained air supports the weight of the shell.
A shellworld13 is any of several types of hypothetical megastructures:
A planet or a planetoid turned into series of concentric matryoshka doll-like layers supported by massive pillars. A shellworld of this type features prominently in Ian M. Banks' novel Matter.
A megastructure consisting of multiple layers of shells suspended above each other by orbital rings supported by hypothetical mass stream technology. This type of shellworld can be theoretically suspended above any type of stellar body, including planets, gas giants, stars and black holes. The most massive type of shellworld could be built around supermassive black holes at the center of galaxies.
An inflated canopy holding high pressure air around an otherwise airless world to create a breathable atmosphere.4 The pressure of the contained air supports the weight of the shell.
Completely hollow shell worlds can also be created on a planetary or larger scale by contained gas alone, also called bubbleworlds or gravitational balloons, as long as the outward pressure from the contained gas balances the gravitational contraction of the entire structure, resulting in no net force on the shell. The scale is limited only by the mass of gas enclosed; the shell can be made of any mundane material. The shell can have an additional atmosphere on the outside.
Having the roof of a shell world supported by air pressure is not some wild fantasy.
An air-supported (or air-inflated) structure is any building that derives its structural integrity from the use of internal pressurized air to inflate a pliable material (i.e. structural fabric) envelope, so that air is the main support of the structure, and where access is via airlocks.
Some quite large structures are air supported, although they are much smaller than air supoorted shells around an entire asteroid or around an entire planet. But any story involving terraforming a planet involves mega projects.
Part Six: Force Fields.
In some science fiction stories force fields of various types might have various practical uses. Perhaps in some stories force fields could keep the molecules of gas from escaping from tiny worlds which don't have enough escape velocity.
Those force shields might make it impossible for spaceships to land or take off. Or there might be giant air locks which stick up through the force shields that spaceships can take off and land in. Or maybe the force fields stop particles moving at the velocities of atmospheric gases, but let spaceships land and take off if they travel faster or slower than atmosphereic gases.
A famous example of a force field holding in an atmosphere is in Isaace Asimov's classic story "Not Final!" Astounding Science Fiction, October, 1941.