Well, I've built my own rack already (and plan on making a second like it), and have the plans and supplies to make a 'bookshelf' style cage system for my corns. I spent a lot of time looking into flooring options, especially since the cages are all connected (meaning the floor of one is the ceiling of another) and I'll be using ZooMed UTHs for the heat and they need circulation. I finally devised a plan where tiles will be set on risers and then silicone will be used to seal the edges.
I was going to use thick glass instead of tile, but it proved too expensive.

If you're using flexwatt, tho, and especially if the cages are stacked but separate units, you can just lay vinyl flooring over the strip. Makes it a lot easier.

The PVC sheeting is available, but very expensive from what I've heard.