i saw one of these pop up on UK eBay some weeks ago, they came on the 54-58 Swingarm Model BSA A7/A10 twins as an option:
seems like a decent compact idea, I think something
like this with the perforated/gauze face at the side of the bike (perhaps under the seat) would be ideal so it can suck in cold air but hopefully face backwards slightly to avoid catching crap directly, the tricky part would be mounting some sort of uni-filter foam air filter in there with a seal as that paper job looks woeful at best, I'd probably have the metal box rigidly mounted to the frame and obviously a rubber inlet tract to the carby would be the best option due to the horrible vibes of the bloody motor.
hhrrmmm.
-nick