Just recently I finished an OWD course, well not that recently, just before that corona thing. After first couple of dives in a three-noob buddy team in maybe 2 meter visibility I decided to make a mirror to help us stay together. It helped … a bit:-)

Used PETG, 3 or 4 perimeters for the body, no supports required. Manual color change for lid logos.

The holes are designed for 3mm bungee cord. It takes some persuading to get it through the bends. Helps to untwist a bit of the cord and cut the rubber part out (5mm or so), making kind of a pointy end. Start at the side where there are only two holes, then finish with the two ends. It is easier to pull the cord from the side as there are directional teeth inside.

I've got mirrors on ebay, just search for “convex car rearview mirror”.

(The octopus lid is not included, sorry, those were printed as a gift for our friends from hvardiving.com)

Currently I'm using it together with the suunto SK8 compass mount: https://www.printables.com/model/70155-suunto-sk-8-compass-bungee-mount 

Update 2024-11-03: added OpenSCAD source files