I hate melamine. I used OSB for my enclosures and sealed them with epoxy and silicone. I paid $9 per 4x8 sheet. It does have a lot of nooks and crannies which can be slightly harder to clean, but it still does the trick just fine. If you wanted a smoother surface you could go with a cheap exterior grade plywood.
Make sure that whoeever builds your cage builds it so it can be dissasembled, as it will be too large to fit through any doors in yor house fully assembled