Summary

I designed this to make my UV Galvo Laser focus easier for me. I did see other versions like this on ETSY and other sites, but I designed this model from scratch in Fusion 360. I'm sure my work is slopy and someone else could do it simpler and in less steps, but it worked for me. Mount the arm and rotate it so chain hangs under center of lens, adjust the Galvo head so the ball chain just touches the object you want to mark/cut, then get the arm & chain out of the way.

Make sure you put the magnets in correctly so the pieces attract properly.

Mount base inline with center of lens to either side and aligned vertically square (to make sure your height is consistent) See photos
Mount arm to base
Adjust Galvo height to get to correct height for your lens (use ruler, measuring stick, focal laser, etc.)
Swing arm so it is under lens
Measure distance from arm to height laser is adjusted to. This gives you an idea of how much chain you need. Trim Chain to measured length plus a little extra (~20mm)
Press ball chain into teeth of arm a little.
Trim ball chain length to get closer if needed
You can press the chain into the teeth more to micro adjust the length of chain. See Photos (Blue is pressed down all the way, Gray is not pressed down as much)
Once you are happy with chain length, I recommend gluing it into place (Use limited amount of glue so it does not run down the chain, like mine)
Repeat process for any additional lenses you have

Then, mount arm for your lens and rotate chain under lens and adjust the Galvo head to needed height (so ball just touches or +/- as you want) of object you want to mark/cut. Once height is adjusted, move arm and chain out of the way (remove or rotate to side)

You will need:
12mmx3mm Neodymium Magnets (two for base and two for each arm, each pocket is 6mm deep) Search Amazon: 12x3mm neodymium magnets
Ball Chain (length depends on lens used, buy a roll or longer lengths to make sure you have enough) Search Amazon: ball chain
Double sided tape or something to mount the base (I used 3m Command Med strips) Search Amazon: 3m+command+strips+Medium
Some CA/Super glue to lock Chain in place and lock in the magnets

I printed mine in PETG, but other materials should be fine.
I also used the Laser to mark the length and lens size on each arm.
You can customize the sizes with the parametric variables in Fusion 360 file. Be aware that changing some sizes will likely break some fillets and chamfers, etc.