I got some T-pins from Joann's and used those to pin my felt in place. That was 8 years ago, and it's still holding strong. If I ever decide to change the color, it comes off easily.
BTW, the advantage of felt is that it comes really wide and covers the entire 4' x 8' insulation board easily.
AND ... for all of you who don't want to splurge on a design wall like this, I used my design wall during my kids' high school graduation parties as a photo wall. I just used (more) T-pins to place and hold the photos on top of my felt. It didn't wreck the design wall one bit, and it really let us display ALL those photos!
So, if you don't want to spend this money on yourself, DO IT FOR THE KIDS!!
BethMI