Some kind of vinyl would work, but you may need to experiment a little with thickness. Stick-on would be great because it would allow you to apply little by little so you can mold it to the front of the helmet. Using some heat would defiantly help. The issue with adhesive backed vinyl is that you only need to apply it to the area you’re sticking it to.
Glue means you’re only applying adhesive to the area you need but you may have to find one with a slower curing time to allow you to mold the vinyl into place. You might be able to use Foss adhesive (or similar) It’s used in leather and is very strong but allows some time before curing.
I feel like if you made a negative of the part you’re molding to, might help to mold and clamp the vinyl into place.