Little thingy to hold a small disc of Polycarbonate in front of your red dot to stop BBs from cracking your lens. I highly recommend using an impact resistant type of resin/filament.

Requirements:
Polycarbonate Sheet (3mm thick cut into discs with 28mm diameter)
M4 Bolt and Nut (10mm bolt best)

Instructions:

1. Print
2. Cut Polycarbonate (You can use the included "Poly_Sizer1.stl" file to use as a template to cut to size)
3. Insert Polycarbonate disk into the protector from the wide end. Do not glue, you'll want to be able to remove if the disk gets damaged.
4. Position wide end over red dot (forward facing, obviously.)
5. Insert bolt and tighten until secure. DO NOT OVERTIGHTEN.

Final update: I printed and used this for a 3-day milsim and experienced no issues. It held onto the sight perfectly. That said, it didn't take any direct hits.
Based on some testing after the game, you can expect it to hold up to lower-joule shots, but if you get hit in the sight by a sniper, or at a bad angle, the ring will likely shatter, so bring backups!