Make a mirror
As well as a pair of spectacles, you will need:
The most suitable spectacles have flat side-pieces, and a neck at the hinge; preventing the mirror both from twisting and slipping backwards, respectively .
You can use a piece of ordinary domestic mirror, cut with a glass-cutter or tile-cutter, but vehicle rear-view mirrors tend to be thinnner, which makes a rather neater job. And you can find one on your nearest road. A slightly convex surface won't matter, except that the glass is more difficult to cut - although you might not need to. If you have a fine-grit grinding wheel, or a belt-grinder, it helps to run round the edge of the mirror, being careful not to chip the glass.
You could avoid a lot of fiddling about by using a dentist-style inspection mirror and cutting off the handle. I bought a round one with a plastic reflector from Maplin Electronics (England) for less than £1. But I prefer a good-quality glass mirror, because it is easier to see whether the face behind the wheel looks 'ugly'. See the warning below.
Suitable wire is sold at model shops for purposes such as model-aircraft undercarriages, and (in England) is called piano wire. If they don't know what you mean then you are probably in a shop that just sells silly plastic models. Anyway, be sure you get spring temper wire - wire that you could wind springs from (with the appropriate equipment). Spring temper means that it is hard, but not so hard that it will blunt your cutters, or refuse to bend. 1.2mm diameter is a bit thinner than a bicycle spoke You may need to alter this thickness depending on the frame of your spectacles.
Note, in the top picture, how the wire is bent: the end nearest your ear is on the inside, pointing outwards (so it won't stick into you); then the middle crosses the outside of the frame; finally near the hinge the wire passes inside the glasses frame again, and continues forward.
A good way to start is by giving the back of the mirror a coating of filler or adhesive, and allow it to set - this will protect the silvering from rust penetration caused by the hardener. Then bend the wire so it fits onto your glasses as in the photos - assuming you ride on the lefthand side of the road in your country. The double bend at the end, shown in picture 3, forms a hook that helps keep the wire attached to your glasses. Note in picture 4 the way it hooks under the frame at the end nearer your ear. You can bash this bend into shape with a hammer and vice, as shown in picture 2, if you have trouble doing it with pliers.
To attach the mirror glass, bend a squarish loop in the wire just a bit smaller than the mirror, as in picture 5. Then stick the mirror on with car-body filler. You will probably find uses for the rest of the tin even if, like me, you don't have a car.
You want to be able to see the mirror through the rightmost area of the lens of your spectacles About 35 mm separation between the lens and the mirror works well for me. More than this, and you won't see so much in the mirror. If there is less, you will have to turn your head a bit to see behind you
You may need to adjust the angle of the mirror when the bond is set, so that you can almost see the edge of your ear. It is best to adjust it by bending the wire with two pairs of pliers just behind the reflector, so you do not stress the bond between the wire and the mirror.
You may also need to bend the wire so its springiness holds it firmly in place on your glasses - but not so tightly that it bends the frame. You can do this with your fingers. If it is wobbly, you won't see properly with it, and it may fly off when you are going fast.
A mirror like this is easy to lose, and difficult to find again. I mostly lose mine when I absent-mindedly take it off in an attempt to look 'normal'. If you manage to keep one for more than a couple of months, you are doing better in the mindfulness stakes than me.
When you start using your new mirror, don't forget to look where you are going. Things in the mirror will seem to whizz about at first, but you'll soon get the hang of scanning the road behind with your new rear-view radar device. Be aware though that it is not unheard-of for drivers to try to 'teach you a lesson' if they think you have manvred without looking behind.