bike farings idea again

since it's attempting to warm up and i'm attempting to fix my bike again, i've started thinking about the front fairngs mod i talked about a lil while back.

basically:

This:

Plus this:


Equals something like this:


now obviously i used MS Paint to "photoshop" my idea, but this more closely depicts what i want to accomplish... now i'm wondering: what medium do i use to make a life size replication of what i want to do, so that i can fiberglass over it and make it permanent? expanding foam in a can? foam blocks glued together? paper mache? clay? currently i'm thinking expanding foam, or paper mache.