Good idea about brushing up the napp when it gets flattened from use.
My design wall is simply an inexpensive flannel-backed plastic tablecloth, less costly than a lg piece of flannel or felt. I got it for $1.00 at an after Christmas sale one year. I take it down from the closet door in my sewing room and fold it to store, as I do not ned it up all the time, especially while working on placemats or other small projects. When I want to use it, I throw it in the clothes dryer for a minute or 2 to warm it up so it will lose the fold lines and be smooth again.
Good luck with whatever you choose - the insulation board sounds fine, just too bukly for my space.