with enough support you can really use any water resistant material that makes contact with the top of tubs.

ive seen people use clear vinyl meant for auto windows (like jeeps) and a wooden frame with good results. if you want to make the frame and shelves entirely out of 3/8 pvc i agree OhhWatALoser, it depends on the size of the tubs. if the tubs are too big and heavy you could get some sagging. smaller tubs would give more support to the shelf above.

what ever you decide to do let us know and put up some pics of the build. i'd love to see how it works out.

good luck!