[mb6606]

I make a simple board out of two 4' x 8' sheets of styrofoam building insualtion. Layed one on top of the other and cut a circle using a cheap soldering iron. I duct taped the two pieces together and use dowels to prop it up. It can also be used standing on end so you can find the true turned shoulder plane on the backswing. You build another half and duct tape together making a complete plane board.
Pretty simple, cheap and lightweight. Not exactly portable or sturdy for commercial use. You could make it sturdier by gluing with contact cement a piece of plywood to the back.