1-Print these two parts at 10% size
2-Insert a mirror into/on the disc holder of your choice, secure with glue or double side tape
3-Attach base to helmet with double sided tape
4-Insert wire or an unforlded paperclip into the base
5-Bend other end of the wire 90 degrees down
6-Insert bent end of wire into the mirror disc carrier
7-Once you have the distance right and can see behind you mark the wire
8-Secure the wire in the base and mirror disc with glue, I use hot glue
9-If(when) you ruin the wire or loose the mirror print another and reattach

It is also possible to heat the wire and insert it hot to secure it to the base and disc. OpenSCAD files are included to edit if you need a different base or mirror carrier disc.

edits:
-I have added a 25.4mm(one inch) stencil so you can easily cut out a disk from a cheap hand mirror or broken mirror piece with a carbide glass scribe.
-I increased the size of the disc to accomidate the imperial size mirror discs, a 25mm disc will also fit so deleted the first posted holder.
-I also added a generic mirror mount that will accept any mirror you like using double sticky tape, just be sure to bevel sharp edges.