I made an Ironman suit out of foam and I was told it looked right out of the movies, I had to explain what it was made out of and no one believed me. You know they have the foamies sheet at Michael's and hobby lobby that are sticky back, I wonder if you could print out the pep files of heavy paper and stick it to the foamies and cut it out and fold it so the papers on the inside, i bet that would make it pretty rigid.
technically you should be able to resin the outside of your foam to make it hard..
just do a testrun with your resin on your foam (some resins can eat up some foames)
then the helmet would be as hard on the outside as a pepped resinned one.
Hmm I know what you mean and I am building mine from cardstock with the resin approach. I know it will be heavier and will be longer, but I want something that is pretty durable and of course, shiny! And yes, I know I said it takes longer, and it does, but I mean it takes LONGER. lol! I am also building a Trooper suit from ToR and the helmet alone has taken me a week to build from cardstock, and the level of detail on it, I am thinking of skipping the filler part to maintain the detail of the gear, or at least the helmet and just do a resin coat on the outside.