I was only planning on doing it for the gap. It was going to be a bigger problem until I realized all the tubs I planned on using were the same height except the ones. My purpose with this rack was going to be utility and I wanted the freedom to put any size snake I might have into it. Luckily a trip to Lowes found me some nice 3/4 inch sheets of styrofoam that should make for the perfect inserts to use over the hatchling tubs (score!). They do only come in packs of 6 4 foot long pieces, which is a lot, but beggars and choosers and what have you. Styrofoam to be placed over the lidded tubs, not just the tub with the snake in it and no lid.
My original question still stands about the size of tubs needed for the hatchlings and adult males.
Edit: thank you! that makes this a little easier.
Another question, this time about heating; I had planned on using 3 or 4 inch flexwatt on the back end of the shelf for under-tub heating, will it matter that the larger tubs for adults will be technically sideways? Like, the long end of the tub being heated instead of the shorter end? I am open to heating suggestions on this. I'm going to scribble down my design for reference here.