My design wall is 8ft x 8 ft, made from 2 of the 4x8 insulation boards at Home Depot or Lowes. It comes in different thicknesses, mine is almost 1 inch which I like cuz I can also use pins on it if I want. It is covered with the least expensive flannel that was on sale at JoAnns in an off white color. LOVE it. It is only pinned on in the back and on sides so that I can remove it to launder it. Recommend that if you do you use flannel, launder it first before you apply to your design wall, cuz once it is cut to size and you launder it later it won't fit cuz it will shrink.
If I could've I would have made it 8x12 but had no more wall space left.
good luck, be sure to post photo of your finished wall
warm quilt hugs, sue in CA