I use take-a-look, and it mounts to the visor on my helmet. I'm on my second one, because I lost my original after it came off my helmet during a roadside repair stop. Now I secure it on with a sturdy twist-tie (like plastic-coated kind, not just paper).

And take-a-look does make an adapter for mounting onto helmets without visors; but I can't vouch for how well it works. It appears to be just a trivial flap of plastic that sticks/glues into the helmet.