The door seems like the favorite but I used the following:

One side of a ping pong table (1/2 only) - I suppose if you had room, you could use the entire ping pong table. Then cut 4 x8 sheet of 1/4in sheet rock and lay that on top. Put it on two adjustable saw horses and there ya go. You can pin it, it stays flat, etc..

Works pretty nice.

Viper