Thanks!
I don't have much experience with things like Bondo or fiberglass, and the more I thought about it, the more it made sense to just use non-toxic materials that were readily available. The sawdust didn't really work too well; I gave that up after the first attempt with it, but white glue and plaster mostly do what I needed (and there are still health concerns, with dust mostly, but I feel much safer this way).
The visor may involve a 35 oz tub of Utz Cheese Balls -- talk about toxic! But I might have to sculpt something in the way of a vacuum-form mold later on.