I do the same as several have done. I went to the home improvement store and got 2 sheets of foam core board. I also picked up a can of spray adhesive. I used yardage felt and glued it to the front and wrapped it to the back of the boards. They were just the right height and so I was able to wedge them in between the floor and celliung. My DH then used 6 screws and fastened the corners and the middle to the wall. Did the middle because they were bowing out a little bit.
The foam core allowed me to use pins. The felt does hold the blocks, but when I get larger pieces, they tend to get heaver and not want to stay stuck to the wall. I also use it to pin fabric up flat when I am drying it after a heavy starching.