Hm...well I could shift things around so that I can put one smaller size next to a larger one...I might just have to build a separate rack for the adults, in which case I'd likely go for 3'wide, 2' deep and 6'' tall since muddoc says it's plenty...the 3' wide should make up for it only being 24'' deep, and won't take up a whole lot of extra floor space (all this will be going into a 12x11' room along with my bed, desk, and nightstand though luckily I have no dresser, but unfortunately I have windows on 2 walls) and at 5' tall to match the other it'll leave space for 10 adults. Sound good?

I wasn't sure about the size because although I own a fairly large adult, I'd never bothered to measure out something 2'x18'' and see how well she fit...it never occurred to me to try. I've been keeping her in a 55gal and that's horrible to keep the temps and humidity right even with three layers of aluminum foil, one of the really strong heating lamps and a large UTH with the water bowl partly on it. I'm definitely interested in reducing air space.

And I'm not using tubs, like I said; it's entirely melamine and plexiglass, a solid structure, not tubs; I tried but my dad thinks it's cruel, and since he's paying for it (I don't currently have the funds to build a rack system) I'm not going to protest overly much, lest he decide to withdraw this sudden generosity--he's all but inviting me to fill it with snakes! XD That's why it's not more than 5 feet tall; it has to be short enough for me to be able to clean out the very top enclosures without needing a ladder! :p

In any case I figure that 18'' of depth is more than enough for babies and juvies, but thought it seemed a bit small for the adults. Thanks for the input!

And nobody answered my original question...is back heat okay in that sort of a system? Will not having belly heat bother the snakes? Would back heat provide enough heat, provided that I put some sort of insulation on the back to both protect the heating elements and retain heat?