The only thing I can say about the 20g is that it's not a long, but if it seems sufficient for him and he's happier in a bit of a smaller tank then I'm sure that's acceptable for him! :]
Setup generally looks nice, if it's possible, you could try to fit another hide on the other end (and maybe get a smaller water bowl if he doesn't soak) to fit it?
If not though, I don't see a huge issue with it. After all, most tub keepers only have one hide per tub.
To keep in humidity you can foil treat it.. I think starmom is the one who has a foil treatment method that she can instruct for you. (I hope it's not someone else.. oops)
:]
Mesh screens won't melt, don't worry.
Edit: One more thing, get some clamps for the screens, they're easier than heavy bricks and having something that heavy could pose a problem for an escapee! ^^