I did about the same thing with a piece of foam rubber 3"x6"x3'. I marked a .5" sq. or 1" sq. grid system on each face (or at least two faces). I could show bending about weak and strong axis and torsion. Of course, the above will not show flange or web buckling, but it is easy to do and fairly inexpensive. I imagine you could build a WF shape from 1" thick foam rubber sheets, 12" wide flanges, 16" deep web, 4' long. You would have to experiment with the type of glue (maybe some form of rubber cement) at the web to flange connection.