Jumping right in here, head first, I use a solid core door.

You may be able to get one cheaply (or free) from your local building supply store.

Mine is a 24" wide by 80" long and weighs about a ton!

Get a flat one, with no decorative cut-outs.

They too make mistakes and sometimes cut the hinge pockets and door knob hole on the same edge!

Put this on a pair of saw horses and you have a flat and sturdy building surface. I turn mine over about once a year just in case.

Common cieling tile, 2' by 4', makes a great work surface that holds pins well.

I secure them to the board with sheet rock screws and turn them over for a second work surface.

Look for a package of damaged tiles and ask for a price break. I got 12 panels for $3.00 once (lifetime supply, or close to it!)

We modelers are really cheapskates, aren't we.