I used a foamular board purchased at Lowe's covered with one of the Fons and Porter design wall (flannel). I really like it. It will hold the blocks without pinning, however, since I move my design board around alot I usually pin them anyway. I haven't done it yet, but I plan to put a black felt on the back so I can switch it up whenever I want to. Here's a picture of mine if you want to see it.
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