Babies do great in 6qt-15qt, juveniles 15-28qt and adults 32(males)-41qt(females).
I know these are the same sizes that are recommended, but there are reasons for it and it isn't just space. The animals feel more comfortable in the tighter space and husbandry is alot easier to maintain in them.
If you don't want to use tubs you could check out something like the Animal Plastics T-8's. They will give your snakes more space and they are stackable so they don't take up as much space as aquariums do. They also are made for reptiles so they will hold temps and humidity better.