My hubby just made me a design wall a few months back and I love it. We used the foam board (insulation board) from the hardware...we used the thicker board, I think it is 3/4 or 1 inch. My design wall is 6ft x 6ft. We bought 2 boards and cut them 3ft x 6 ft, covered them with batting which was cut to fit and only covered the front of the board to the edge. (glued that down with glue from Walmart, can ask my hubby if you need that info).
We then covered the batting with flannel and that was cut to wrap around the edge of the board and cover the back about 3 inches all the way around. I tacked the flannel to the front of the board with thumb tacks and then flipped it over and we glued the 3 inch flannel edge to the back of the board with the same glue. We spread it with a sponge paint brush and then just pressed the flannel to it and it stuck held and dried very well. After it was all dry my hubby screwed both pieces of the board to my sewing room wall with 3 long screws along the top piece and two down the side. The bottom board had 3 screws along the bottom and two up the sides. Hubby bought large washers about as big around as a 50 cent piece to use to keep the screws from being pulled thru the coreboard. He even painted the washers the same creme color as the flannel and after screwing it all to the wall he got a piece of paper and made a little hole and spray painted the head of the screw so it all matched. You barely notice where the screws and washers are. The design wall works great and I am really glad I used the batting behind the flannel to cover the boards. I will take a picture and post it as soon as I can if anyone is interested. I just love it and when I am not using it for quilting I use it to put up photos of my grand kids with a few stick pins. So it serves as a big bulletin board too.