There's nothing inhumane about using tubs. In fact, after using both tubs and aquariums, I find that my snakes do better in the tubs. They are actually more active in the tubs, but they don't crawl around the edges constantly trying to escape the way they do in aquariums.
For vertical space, you might try using a taller tub, such as the tall Sterilite boxes from Wal-Mart, Target, etc. My brother has ratsnakes in tubs like this and he attaches dowels and branches using screws drilled through the plastic so the snakes have plenty of room to climb.
If you're handy, you can build a rack yourself to get exactly what you want. Here are some pics of a 10 level rack I designed and built for 41 qt.Sterilite tubs. I houses some of my milks and kings. The shelves are made from pre-cut melamine shelves from Lowes. I added a 1x2 spacer board along each edge to make the shelves wide enough to fit the tubs and also to help stabilize the whole thing (so it doesn't wobble).
Good luck!



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