An atmosphere at STP is just a bunch of gas molecules bouncing off each other. On Earth they are prevented from flying off into space (most of the time) by gravity. You need some other force acting on the gas molecules that try and bounce away from your space ship.
There is no current way to do this known to science, especially not with your caveats.
Our only two options are to use electromagnetic or gravitational fields to apply our force.
It might be possible to have a dense layer of ionized gas molecules held between two very strong electric fields. When non-ionized gas molecules bump into the "trapped" ones, they would just bounce back into the containment area.
But oops, gas molecules don't always collide and can move right past each other, and do so many millions of times per second. This means our ionized barrier layer needs to be dense enough to stop almost every molecule from escaping, which would definitely be noticeable. Similar to how a balloon's skin or an aquarium's walls are noticeable.
Even if that was possible it would be insanely complex to have two perfectly counter-balanced electromagnetic fields held in a spherical shape, and would not be "energy friendly".
You could simplify that approach by ionizing ALL of the gas molecules in your atmosphere, removing their electrons. Then a single, extremely strong negative electric field would attract all the positively charged gas to your ship. This would present a whole slew of problems, from interfering with electronics and communications to degrading the material of your spaceship as electrons are forcibly ripped from them. This is not a viable idea by any means.
Another theoretical way to do this is with artificial gravity fields. Right now the only way we know how to generate gravitational fields is with a lot of mass. Having a super dense point of mass, such as a small black hole or a neutron star, could create the required field strength to trap an atmosphere, but would also collapse any possible support structure you tried to build around it. This would be super impractical to haul around as well, since a glass shell around your ship would weigh far less than a literal black hole.
Electromagnetic fields: No way to do it reliably, cheaply, and in an unnoticeable way.
Gravitational fields: No way to do it without literally being planet sized.