I used laminate pop-together flooring and put the vinyl wall trim for bathrooms zip tied around the inside edge to keep in stray poops/hay. You can put down washable rugs/cardboard for traction but mine don't seem to mind the laminate floor.
Coroplast can usually be found at sign making shops so call them and let them know that you want it for an animal cage instead of sign making (sometimes they up the price for other sign makers to eliminate competition).
I used a shower stall base. I know it's rather bulky and slightly unconventional, but it is super easy to clean up messes and under the plastic is foam, which provides a nice layer of insulation. I found mine at Home Depot in the shower/bath isle.