The biggest challenge with these was getting the mirror in the right orientation.  Being limited to PLA meant not having adjustable screw points like shooter glasses.  Instead, there was much melting & shaping of PLA to get the orientation close, then modeling permanent PLA parts. 

The result was what lions would consider superior to commercial rear view glasses, at least for watching robots on the ground.  The robot still has to follow to the left to be seen easily.

Mirror material came from a dollar store phone case.  It has to be melted rather than cut.