I'd actually recommend Dura-Glass (Fiberglass-infused body filler)... However, you are going to want to take some 36-grit sandpaper or rough emory cloth (about the same thing) and some final-wipe/wax & grease remover/anything to remove oils (but not melt the plastic) to the area to be bonded for maximum "bite"... Your helmet isn't going to see the same vibrations, flexing, etc. as your motorbike fairings, and Dura-Glass is some tough stuff when given a properly prepped area! We used to use it on metal, plastic, wood, etc. all the time at the body shop I used to work at.... Yes, wood.... Some people have some silly projects they bring in for finish and paint sometimes.....
Oh, and speaking of plastics and body filler, there is a certain glazing putty developed for urethane bumpers; it has good flex to it! I would suggest this for the final fill, though with proper prep, regular auto body glazing putty should work just fine as well... Just remember to prep EVERY little crack; red scotch-brite pads work well for getting a good scuff in the hardest-to-reach little crevices.... Just don't scrub it like you're trying to get a casserole dish clean; just enough to make the area dull, and you're good.
Also, Loctite makes some wonderful 1-minute and 5-minute epoxies that I highly recommend! Again, though, you will most likely want to rough the surface before applying, and these epoxies seem to like being applied to both sides and fused together, so to speak... If your seam is too tight, you could drill some 1/8" holes through the seam and moosh the epoxy through it, making sure both sides of the seam are covered and the epoxy flows through said seam (via the holes, etc)... Just make sure you sand and fill and finish the outside correctly afterwards...
But then, I've never built a helmet; I've only ever used these bonding methods on cars and scale car models... And some other miscellaneous plastic/metal props, but nothing quite as complex as a Boba Fett helmet...
Also also, I second the PVC cement mentioned above. I've not used "Goop" glue, so I can't really comment yea or nay on that product, unfortunately...
A note on bondos/glazes, from softest/most flexible to hardest/least:
Urethane bumper glaze
Glazing putty
Light body filler
Body filler (Bondo-brand filler one often finds at Walmart, for example)
Dura-Glass
Lead brazing
Glazing putty can be applied very thin (often 1/8" or less), but normal body fillers will need a good thick application , or it'll simply crack up and fall off... You can take this further with Dura-Glass, as it needs a good thick application or again it'll fail quickly...
