Rearview mirror attachment for sunglasses for use on bicycle/scooter or in sports (tactical advantage in ASG/Paintball? Opponent approaching when running?) - let you have rear view in the upper left of your field of view without turning your head all the way back.

Simple print, simple build but may not be optimal for your head shape, so there's a parametric FreeCAD project attached.

Artifacts:

• Glasses/sunglasses with frame broad enough to attach the mirror from top. The ones I use are linked below.
  • Small adhesive acrylic mirror sheets
  • 1 M3x6 (or a bit longer) screw, 1 M3 nut.

Print the mount
Draw around the flat part of the mount on the back side of the mirror (paper), then cut deep with a snap-off knife, extends the cuts edge-to-edge and the mirror will neatly snap along the lines. Detach the paper and affix to the mount.
Drill, or melt a hole in the top of the glasses frame, as in the photo
Affix the mount using a screw and a nut, not very tightly.
Put the glasses on, turn the mount till you -almost- can see see your own ear. Tighten the screw.
Don't be discouraged by small FOV indoors. It gives a decent rear view on the road, and you can always turn your head a bit to see more.

Affiliate links:

Sunglasses with wide frame
Acrylic mirrors
M3 screw set