I'd go with a rack for sure. It'll make maitenance so much easier as your collection increases. Also, your snakes will feel more secure, which may help them eat better.

On the humidity issue, what are you using for ventilation? I use a pencil saudering iron and melt holes in the side of the tub like this:

O___O___O___O___O___O___O___O___O___O___O___O___O___O
__O___O___O___O___O___O___O___O___O___O___O___O___O___O

etc. (O=hole, ___= space)


I generally leave about one inch of space horizontally between each hole, and I have holes all the way around the tub. That should definitely keep your humidity down.