Helmet is looking pretty freakin' awesome!
I know you're still working on that buck (so you may already be planning on doing this), but there are a few things you may want to do to the box part to make your life easier before you make your first pull. I'm no expert, but I have done my share of vacuum forming for work and there are some things I've learned along the way.
It looks like you've got some massive undercuts going on where the bottom of the visor overhangs the box...which means you will not be able to get your formed plastic off easily once it's pulled. Since you're planning on pulling multiple visors, you'll want to be especially sure that the buck slides out easily from the formed plastic...otherwise you'll be wrestling with it after every pull and may even damage your buck trying to get the plastic off (not fun). Removing all undercuts and adding a gentle draft angle to all sides of your buck will definitely help with that. You should also probably add another 1"-ish to the bottom of your box to get your visor off the table. It's generally not recommended to have your desired shape extend all the way to the bottom of the buck...the pulled plastic can sometimes do some funky things around the bottom edges of a pull. Just my 2 cents.
Anyway, keep up the good work and sorry for my blabbin' if you already know all this
I was planning on filling in the gaps by sealing it partially with fiberglass or MDF board, and then filling with rondo (for strength) and smoothing with bondo (for pulling), I forgot to mention that the above image is just the beginning of the work on the buck, but I'd rather be warned of things than not, I tend to flake out about lots of things for instance...
I did not know about the extra 1" thing, I'll be sure to extend it when I work on smoothing out the box part. Your insight is greatly appreciated, I'm sort of flying by the seat of my pants, so any tips are awesome.
It will be tinting and detailing that are gonna kill me I think. I want it to be totally opaque from the outside, but still well visible from the inside, most spray-on tints I've seen here end up with a foggy interior. Going to experiment with stick-on tint, mylar, and I dunno what else... bleh.