old thread alert, but ...
I have a very functional
Taiga Cycle Helmet Rain Cover
for riding in the rain, thanks to a fellow BF contributor, who recommended it to me, years ago. folds up pretty small & very fast & easy to put on. very well made. the front brim, side & rear flaps really help a lot! it only looks dumb if it's not raining. when it is raining, you are the smart one ;-)
bought mine on Amazon but they don't seem to carry that model anymore. maybe buy it directly from the Taiga link above?