I have built racks out of 1X's and hardboard (Masonite). I keep lids on my tubs so warping is not that big of an issue. I build the frames out of 1X1 and top em with hardboard. I use 1X1 and 1X4's for support legs and it runs fairly cheap and is light weight. I am getting rid of all of my Melamine racks and switching to this design to save on weight, cost, and space.

My rack I built for my boas which holds six 74 qt tubs cost about $35.00 in material. Tub cost was another thing, but still cheaper than trying to build individual cages or more elaborate rack systems.