The Bike Eye mirror fits at the top of the down tube.
The big problem with this location is that your view to the rear is obstructed by your legs. Yes, you can stop pedalling and cock your leg out to the side, but that limits the use of the mirror to specific checks, rather than just keeping an eye on what's happening behind, and the leg out to the side obstructs the view of anything closer. Plus, of course, there's the matter of any luggage etc. that may also obstruct the view (saddlebag, rack pack, coat).
With flat bars, a bar end mirror will work well.
The Mountain Mirrycle
is one of the better versions, and can be set so that it both doesn't move in use, and can still be pivoted out of the way for parking.
For drop bars, the only really good mirror is (again) the (road) Mirrycle
, which is even better than bar end mirrors on flat bars, but is, unfortunately, only available for some versions of the Shimano STI lever, or the original version
for old-style non-aero brake levers with the cable out the top (the latter version can sometimes be bodged* to fit other levers).
I find that on drop bars, any bar end mirror is behind my arm when I'm in my normal riding position, which limits use to specific checks rather than keeping an eye on what's going on behind (besides which, my bar ends are occupied by gear shifters). Having said that, bar end is probably the best relatively easy location.
* Note to myself - since I've just replaced a damaged RL520 lever, I'll have a go at bodging my old Mirrycle Original to fit.