You can use anything that is dimensionally stable and will not warp. No matter what you use make sure its flat. If you go the hollow core door route make sure you check it with a straight edge. A lot of them aren't flat because the core is really a honeycomb of cardboard. I'm using a table top made of laminations of particle board that is 3" thick. It's covered with a laminate and I spray glued a ceiling tile to the top. The tile is smooth surfaced, it has no texture. It lasted me about 10 years before I had to replace it.