Those of you who have never needed to wash the fabric on a design wall must not have cats!
When I had Warm n Natural batting on my wall years ago, I did have problems with blocks falling off it. I think it may be because my dh made the design wall flat against the wall. I have since thought that covering styrofoam from the hardware store would work better since I could prop that against the wall and the batting would then not be straight up-and-down; the slant would help keep the blocks in place. Ideally I would "box" the styrofoam with lightweight wooden strips which would extend beyond the styrofoam to create legs. This would lift the styrofoam off the floor so I wouldn't have to bend down as much (and also so the cats would not be brushing against it all the time!). I like the idea of having a moveable wall too.