Yeah, call around to local plastics manufacturers, and see if you can find one that sells expanded PVC board. It can be worked virtually identically to wood, and it's moisture proof and can be disinfected.
It's not tremendously more expensive than wood. If you have to go with wood, try to order some melamine board that's made without formaldehyde.
You're going to have to put some real thought into how you're going to heat these.
(alternatively, consider Vision cages--they're stackable and look really nice).