K40 Laser Mirror Alignment Tool - Customizable

This is a remix of DIY3DTech's:
eBay Chinese CO2 Laser Cutter & Engraver Mirror Alignment Tools
https://www.thingiverse.com/thing:1149832

To improve on the original design I did the following:

1. Redesigned it using OpenSCAD.
2. Measurements can now be adjusted using parameters.
3. Added an opening behind the target for aligning the tool with the mirror.
4. Target frame is now a circle and can be the same size as the paper target.