I'm not quite sure I understand your post.

Rack systems are not any "smaller" than that tub you got. Most breeders/owners put their adult bp in a CB70-size tub in a rack. Floor dimensions are: 33"X17" which is bigger than a 20L glass tank.

If you're talking about the height of the tub - that doesn't matter much to a bp because it is a ground-dweller. It doesn't habitually climb trees like GTPs do. So, a 5" tall tub is not much different than a 12" tall tub for a bp.

Your set-up is okay. He'll outgrow that tub, so you will need to get a bigger one as it grows.

BPs get stressed when it is "out in the open". Like you said, your bps stay in its hide most of the time - that's just the way they are. So a hatchling in a 20L glass tank does not feel as secure as a hatchling in a shoebox-size tub.