Update!
We got our vac table working tonight, and we just made our first two visor pulls. We used some .04" PETG plastic - rather thin, but it formed well and once shaped into the ODST visor, it has pretty decent strength (though, it's not like it's load bearing or anything anyways...). So I popped the first pull into my helmet, and here are the results:
The buck isn't the ideal shape for vacuum-forming. It should be a mound-shape, with supporting sides leading up to the edges of the buck. But this worked well enough for our needs, and because the plastic is thin enough, it can just be trimmed off the buck with a hobby knife.