The scrap piece I had was 70cm x 25cm (about 29" x 10"). The core board was 5mm thick. This board is used for mounting photos and other artwork, I bought it at an art supply shop. It has a foam core with cardboard on both sides. I made the sides 20cm (8") long. I marked a straight line at each end 20cm in from the edge. I cut the board using a heavy ruler and a craft scalpel knife. Don't cut all the way through, you are aiming to cut about half of the way through the thickness of the board. Once you have made the cut just bend the 2 sides together and the foam will snap a bit and make a good corner.