Remember those vinyl table covers that go on sale every spring/summer? Many of them are backed in something (not sure what on earth to call it) that would work great for a larger design wall. It's some kind of white felt fuzzy backing. Anyhow, it works great to plan blocks on. They also make great flannel boards for kids as well.
I got one of these tablecloths at Walmart for $1 a few years ago. Works just fine.
“Anyone can give up, it's the easiest thing in the world to do. But to hold it together when everyone else would understand if you fell apart, that's true strength.”