You can have people 3D print the message on their personal and business printers. Most plastics never biodegrade, except in very specific conditions, so they should last for 50,000 year. As other answers suggest, having multiple copies will make survival of the message more likely.
Granite, slate, ceramics, concrete, and other hard natural materials can be milled or laser engraved/cut with the message as well. Many maker/hacker-spaces have CNC mills and/or laser cutters capable of doing this, as well as private businesses. It would only take a trip to the local home improvement store to get a stack of pre-made 10-12" square tiles to get this project crowd sourced, same as the 3D printing.
You can also mill and/or laser cut sheet plastics. Acrylic is cheap, flexible, stronger than glass, and readily available. Polycarbonate isn't as laser friendly, but will still etch and is tougher than acrylic. Both can be sourced as 4 ft x 8 ft sheets.
Crowd sourcing is probably the best way to get a large message like this together in a short time, and it gets around the so called "one shot" you think you have time to make. Giving small sections to a variety of people with specific instructions on how to make the end product will give you the quickest results. To hedge your bet, you can give one group instructions to do a set in laser etched polycarb, another group CNC mill engraves it into granite headstones, another group is told to 3D print it, etc. Once the individual pieces are finished, they are co-located and assembled into the finished product, with each groups efforts being distributed. What I mean by that is maybe the headstone groups is in Los Angeles, the 3D printer group in New York, and the polycarb group in Denver.
You can even have this duplicated in each continent, to hedge your bets even further. So, for example, the 3D printing is also done in Melbourne, Paris, Moscow, London, Capetown, and Rio de Janeiro.
Each continent or location can have their own translation, so it becomes a Rosetta Stone, in case one or more language is more easily decipherable by it's finders than another language. Some of these translations can include pure math, binary, pictorial, as well as current spoken languages.
Also, remember to number the pages, somehow, so your message isn't garbled when the cargo container is ripped apart and spread across 3 square miles of ocean floor. Or whatever.
With potential dozens of these messages around, someone is likely to eventually find it. Someone will do some spelunking in caves that used to be Buddhist temples, find perfectly straight holes in the ground with radioactive rust piles at the bottom (missile silos), and other locations were the message copies are stashed. A reasonably smart (or lucky) being will decide that all the sheets of plastic (and piles of oddly shaped rock) that have etchings on them mean something, will want to get them into one place, and may even start to figure out that there's a real message in all that (presumably) purposeful surface scratching and/or raised ridges.
Edit: Units are there for a rough comparison of size. I doubt that a country that uses SI units is going to have products that measure exactly 4 ft x 8 ft panels, as that comes to oddly specific (and fractional) 1.2192 m x 2.4384 m. I would hope they round to something "normal"/"useful"/easy-to-measure like 1.5 m x 2.5 m. Or maybe they use 2 m x 2 m panels. I haven't lived in these countries, so I don't know what size building materials actually come in. So, I don't see how converting from UCS to SI units really adds anything meaningful to this answer.
Edit 2: Polycarb is more scratch resistant than acrylic, and can have a scratch resistant coating added, like they do for glasses lenses.
Also, I don't see how people would use table tennis balls for this message, since nitrocellulose is the "spontaneously combust"able plastic Mark is talking about in his comment.
Acrylic is more temperature sensitive than polycarb, and acrylic can be used without warping up to some pretty extreme temps for the surface of the Earth. "It can be used continuously in a temperature range of 170-190°F. It begins to soften between 210-220°F and starts to melt between 300-315°F..."
Just because it's called "organic" doesn't mean it's found in nature. Organic solvents are man-made and have a low boiling point, so even if they were naturally occurring, they would boil away too quickly to do any real damage in even room temperatures.
UV light can damage some plastics, but it can also be hardened against the effects.
"Many of us benefit from UV radiation-cured protective polymeric coatings, such as polyurethane-acrylates, on exterior automobile components."
Some plastics aren't as susceptible to that damage as, say, polypropylene.
"Unmodified types of plastics that are regarded as having unacceptable resistance to UV are POM (Acetal), PC, ABS and PA6/6. Other plastics such as PET, PP, HDPE, PA12, PA11, PA6, PES, PPO, PBT and PPO are regarded as fair. Note that a PC/ABS alloy is also graded as fair. Good resistance to ultraviolet rays can be achieved from polymers extruded by Zeus such as PTFE, PVDF, FEP, and PEEKTM. The only plastics found with excellent resistance are the imides, Polyimide (PI) as used in the Hubble Space Telescope and Polyetherimide (PEI)."