I build dollhouses and miniature room boxes so I use a lot of gator board. It is 1/4' thick with foam in the middle. There are two kinds, one covered with cardboard and the other in a VERY thin wood. We use the wood one.
It's very easy to cut. You just score both sides ans snap it over the end of a table. It comes in several sizes but we buy it in 4' x 8' sheets. Italso cuts esaily with an electric saw.
I make a lot of purses and tote bags so get a lot of "bottoms" from a sheet. I make a "pillowcase" that is removable for washing.