I finished my design wall this morning and other than being a little cumbersome to work on, it went together surprisingly well. I began by gluing two 40"x60" sheets of 1/4" thick foamcore together with Gorilla Glue, then taped the edges together with white duct tape. Then I used spray baste to position a crib size cotton batting layer (for pin-ability without puff). After it was smoothed out, I turned over the board and taped the edges on the back with more of the white duct tape. I had though of using hot glue to attach the flannel, but the tape was sticking so well that I used it to wrap the board and tape down the edges on the back. A few more reinforcing strips at the corners and voila!
After I photographed it, I realized a picture of a big white rectangle was not exactly spellbinding, so here it is holding my latest small quilt: