[gooberific]

If it's on your floor, I would just use a shower curtain or the trash bags, then use the absorbent underbedding mentioned, such as mattress pads, comforters or towels, and then top it with the fleece.

If the cage is raised and you have the grids along the bottom to support the cage floor, I would really recommend getting the coroplast (campaign signs are so great for this, really) as soon as you can, or use linoleum... the sheets work better. I had a heckuva time with the tiles trying to create a waterproof bottom. In the meantime, If you go ahead with using the plywood because of your cage being raised up, make sure you put a waterproof layer on top of the plywood.