ORIGINAL: Drexus
Exterior Insulating foam slabs. The kind you use around the concrete walls of a basement. This "SM" Foam is absolutely strait, never warps. Loves pins and knifes, light, cheap, lets you cut it down to any size. You can get it in a 2" thickness.
I use this stuff too and I use an MDF board with a laminated surface for the actual flat building board.
You can get both at the Home depor or Lowes or whatever good local store you have.
I think I spent about $12 total and the foam was wall size and I cut it down to four slabs for building on. A slab will last through a few models easily before it gets a little too dented from my elbows and hands...