I'm using 1"x 3" frame with three supports between the Boards. 2" foam it's light weight.
This is the end of the layout showing the 2" foam set down inside the frame.
This is the same way I build my first layout in 2006 and it's holding up good I had attached a piece of door board 1/16" on the bottom after wiring the switches. It than sets on a table.