I personally go for the shoulder twinge, and find that sufficient to get a sufficient view of what's behind.
I know Jack Lilleys do make/sell mirror extenders for these, but from the picture on their website I didn't like the look.
Try:
Jack Lilleyonline Store
I'm sure that other accessory companies do similar stuff, though I don't know of any.
Another solution is to go full throttle everywhere, and nothing should sneak up on you...