If you use either corroplast or cardboard, I'd cover it with secured grass mats (so they don't slip) and replace them when they're eaten.
I prefer thin plywood or any thinnish board. I think I have thin oak on Mikey's shelf).
The rabbits love traction more than just about anything (other than cover over their heads).
I have a vinyl mat on Mikie's floor (in an amazing cage built by Bunny Rosanna's Robert), which is actually the back side of a rug. The frame has wood built around it so they can't pull up any sides or corners.
sas :bunnydance: