This is a retention ring for a clear security camera dome. I created this for an allsky camera project.

I've included the FreeCAD file for this so you can modify it for whatever dome you have. I made this for a 3.1" dome I got off of Amazon. (https://www.amazon.ca/dp/B0D1LLJ6CQ?ref=ppx_yo2ov_dt_b_fed_asin_title)
Bolt holes are sized for M3 bolts.
 

I've added another version of the dome with a wider ring and 8 bolt holes.