a door can be used as a flat building surface, topped with a ceiling tile. Most guys here seem to use a hollow core door because they are inexpensive. In my experience, a solid core door is usually better because it is more flat. But, solid core doors can be expensive. What I have done is use a hollow core door, but then use shims under the ceiling tile to flatten it out a bit. This works well and is not too expensive. Others I have heard use a sheet of 3/4" glass. That is something I would like to look into. Not sure how much the glass would cost.