There are alot of alternatives to bondo. Clay, Apoxie Sculpt just to name those 2.
And personnaly, I don't how you would resin and not bondo. Fiberglass is way harder than bondo, and just as toxic I believe.
Resin, Fiberglass and Bondo.
If 40$ for a respirator is too much for you, I don't see what you'll do with a paper helmet with resin on top of it.