AFAIK, a silencer is essentially a double-walled tin-can filled with some form of metal mesh or other way to divert and slow down the gases (like baffles or valves) between the walls and holes to allow gas into that space.

I think you’re right on needing a larger design, as I remember the one I saw had pretty thick walls and was a single piece design meant for printing in PETG, but I think heat is more of a concern rather than pressure, because you still want the gas to exit the front of the gun, you just want it to take longer to do so by traveling through more space before exiting the barrel. The one I saw looked larger than what I would expect a silencer to be, but probably only by about 50%, and I believe it was designed for an AR-15, which is usually chambered in 5.56, which is a projectile that’s just a tiny bit wider than a .22, but has more gunpowder in a longer casing for a faster projectile. I believe the Mythbusters back in the day did an episode where they proved that even something like a potato or a pillow does work as a silencer for a pistol, if only for one shot.

I believe that glocks are usually chambered for 9mm, which is a bigger bullet but has a comparatively small amount of gunpowder for the size, and if he was using subsonic ammo, that would reduce the gunpowder even more. If that’s what he was using, it would stand to reason that he wouldn’t need as sturdy a silencer for the gun (and would also explain why he had to manually cycle the gun after each shot as there probably wasn’t enough pressure to fully cycle the gun normally). But it’s all conjecture anyway on whether or not he actually 3d printed a silencer because it’s too hard to tell from the footage that we have without knowing specific 3d printed silencer designs that it could be.