For mine, I'm thinking of using this leather-like material. I'm not really sure how to describe it. I made most the strap of my 4th squad bag out of it: http://www.cosplay.com/photo/1801650/
I had put craft foam on top, but it has gotten quite a few dents in it and I'll probably remake it without the foam next time. So, I don't think foam is a good idea. As for the lenses, the best thing to do would be to find actual bike goggle lenses. I don't think acetate would look quite right.