My sister-in-law (who happens to be an upholsterer by trade and is visiting this weekend) says to use standard door panel backing board, which is basically a hard, high density cardboard material. She says it should be available from your local upholstery shop in rectangular sheets.
If you can't get that, you might also try your local building supply store, and see if they have 1/8 inch masonite. It's a bit harder than the cardboard material, a bit less flexible (but GT panels are fairly flat), and a it more difficult to cut.
HTH
If you can't get that, you might also try your local building supply store, and see if they have 1/8 inch masonite. It's a bit harder than the cardboard material, a bit less flexible (but GT panels are fairly flat), and a it more difficult to cut.
HTH