Recently I had some minor issues with my Shenhui 50W eBay laser machine. Laser beam was not reaching 3rd mirror in the center, so I had to re-align 2nd mirror to make corrections.  Normally this is a long and tedious process of moving laser head to different positions, attaching sticky tape, firing laser and then trying to figure out if it hit in the center or off, adjusting mirror screws as needed and so on.

For me hardest part was finding center of the 3rd mirror opening, as it was covered by tape. After few pulses tape had to be replaced.  So with help of my Prusa i3 3D printer I made this “jig” that holds piece of paper between itself and 3rd mirror mount.  You always see where center is, there’s no mess left by sticky tape, and it’s pretty easy to attach and remove.

Head without jig

This jig fits my SH-G350 machine, with 3rd mirror holder 34.3mm wide and hole opening of 18mm. You might need to adjust model to fit yours.
Just cut a piece of thick paper/card 40mm x 33mm and place between jig and mirror holder. Jig just snaps onto it.

Head over to Thingieverse to download model files.