I bought a ten dollar fleece blanket from a housewares store and it's great as a design wall.

I went to Goodwill and found a queen size flannel sheet that has moose and bears on it as a design wall. I have found it works great for me and it only cost $3.00.

I started with a flannel-backed tablecloth. Too small.
Then I went to a king-size white flannel sheet. Worked okay, but blocks would fall off any time someone walked by. I finally tacked up a large piece of batting. Nothing falls off, it works great!

I have wood sliding closet doors. I covered them with 2 layers of warm and natural, then cream flannel. I love them!!