The thinner the styrofoam, the more likely it will snap as you move it about. Plus you do put weight/stress onto it as you press your pieces to the wall.
Do you have a set of sliding closet doors? Perhaps put your flannelette onto them?
Also, there's a flannelette fabric that comes with a grid on it, that works perfect for design walls.
My design wall is fovered with black polar fleece .... fabric pieces, actually whole quilts stick to it nicely. I chose it so I could get the 60" width. Oh how I love love love my black fuzzy wall!!! :)