I just bought a cheap Bar End mirror and mount it on my bar extensions. Doesn't stick out, and tells me if there's a car behind, when I want to turn right (we drive on the left here) and it saves my ageing neck..
It's well adequate. Would be better mounted higher, or with a stalk, but that would vibrate; as it is, by the time I've finished checking it, I'm usually uncomfortably close to the kerb. . . .