Hollow door does indeed work well as a flat surface to work on. I got a nice closet door from my local hardware fot $35 and have been using it for years. Pins will not go into the door itself, I put a sheet of 3/4" Dow board (foam insulation board) on top of the door, works very well.
A bag of lead shot from a local sports store comes in handy as well. You can put some in a baggie to weigh down curved areas while glue dries and is is also nice for when you have to add weight to balance the plane (you should always try to balance by moving stuff around and only add weight if absolutely needed)