Cheap hairnet seems like the best solution, assuming it can actually stretch all the way around a helmet, which is a LOT bigger than a head.

My son caught a bee in his helmet vent and got stung a couple weeks ago, he was freaking out that he couldn't get rid of it, when I got to him the bee was struggling to get away from his guts that were embedded in the scalp. The sting wasn't that bad itself (fortunately my son's not allergic), but the uncertainty of why that bee won't go away, and does he have friends, etc etc...