I use a hollow core door as a base like most mentioned here, but instead of ceiling tiles, I use a piece of 1/2" drywall cut to fit the size of the door. It's heavy and lays flat without needing to be glued to the door but accepts pins very well. You can also write on it to make notes, jot down lists, etc. You can cut on it as well, and after one or two kits being built on it you can just throw it out and get yourself a new piece and have a brand new board to build on again.
The only thing I do to the drywall after being cut to size is to seal up the cut edges with some packing tape, just to keep the stuff on the inside from coming out.