I have a 4x8 foot table that was built for a model train layout. I have placed a heavier than normal hollow core door on top of it (The door is 1 3/4inch thick, while most commonly they are 1 3/8), which is shimmed to stay flat and level. I actually rescued the door from a neighbor's trash. On top of the door I use a piece of 1/2 inch drywall. The drywall holds pins well (sometimes it takes a little to push them in, but they stay) and it is very easy to make clear marks on it as needed if you are not building over plan. often have been able to get a damaged sheet at the local Home Depot for a few bucks. I second the note to seal the edges with tape to keep the dust in.