What I use is a plastic folding table with metal legs I bought at Lowe's. On top of that I place (no glue) a hollow core door, and then on top of that I use a 1/2" piece of drywall cut to fit the dimensions of the door. This is just placed with no glue on the door.

It works great because the weight of the door and drywall it lays perfectly flat and you can push pins into the drywall, plus since it's paper-faced you can write on it to scribble notes, dimensions, etc.

Mine usually lasts for about 1 or two projects before it gets too cut-up, etc, then I just simply buy a new piece of drywall and I have a brand new table top to work off of again.