I contemplated the same question until I read a tip for cheap motel style shower caps. they are great. thin, easy to store, easy to fit, let your helmet colors reflectivity and design show through. they work great against the cold and wind too. when things heat up it's so easy to whip them off at a traffic light and stuff in a pocket. no struggling with a tight fit. no giant goody yellow ball on your head.

of course you need to stay in a decent hotel first to get one hahaha but maybe you can get such an item at cvs?