I just finished this x-pen set-up last night!
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.
Thanks for letting me share my project!