They're all a bit convex. A flat mirror wouldn't work very well. (DAHIKT)
First and foremost, do you have the end of a handlebar to mount it to? If not, it will severely limit your options.
I've got the larger ?80mm B&M Cyclestar on my Streetmachine (USS with bar-end shifters), on account of the band-on mounting option.
A pair of smaller Cyclestars on the bars of the Red Barron (tiller steering), again using the band-on mounting, inboard of the brake levers. This puts the mirror above the hand grip, peering over my shoulder, and doesn't add any width to the bike. More aero for racing, innit.
Barakta's trike was supplied with a Myrricle, which is an excellent mirror in the style of the larger Cyclestar, but bar-end mounting only.
Decathlon do cheap teardrop-shaped mirrors with bar-end mounting which are optically excellent. Not sure about the longevity of the mounting (I've bodged a couple onto the mudguard supports of the above trike using a ball stud of appropriately unobtainium diameter from a supplier of car boot gas-lift thingers).
