The base was supposed to be 4x4', but I had my wood cut at Home Depot and I discovered the guy was off in his measurements when I got home. It turned out to be okay, as the x-pen fit around the plywood instead of on top of it. I'd say this plywood is 4' x 47.5". X-pen came from Amazon.com.
On top of the plywood, I laid 16 vinyl peel-n-stick tiles.
Made an inside bumper out of stock lumber to prevent chewing on the edges of the vinyl tiles and to keep litter/poop pellets from rolling outside the enclosure.
Bought a bound carpet square from Menards for $1 for an "area rug."
Built a sitting platform and ramp using the other half of the plywood, a 1x2 and a 2x2. I had the Home Depot guy cut the extra plywood up into various sizes and widths to make it more usable when I got home.
Menards also sells indoor outdoor carpeting by the linear foot. Got some of that and some carpet tape for the ramp and platform.
This is my 3 level bunny mansion, it houses a female polish, and a male holland lop. Made from NIC storage grids and wood, bottom has vinyl tiles. Top level also has foam mats. They love all the space!