This is a replacement lens cap and lens cap retainer for an ATN Night Spirit 2 night vision monocular.
I modeled the holes on the retainer and cap to use 1/8" shock cord to tether them together. The lens retainer is designed to be held on with one or more zip ties that fit into the recess. Additionally, the retainer is split at the top to enable you to spread the ring apart to slide over the body of the device.
I would recommend test prints at low infill to verify fitment, as the tolerances are small to fit tightly on the device.
UPDATE Dec 03, 2018: I enlarged the holes on both the cap and retainer to make them easier to thread the shock cord through. I also added a V2 version of the cap with more rounded edges, and a V3 version with much more angular sides.
UPDATE Dec 04, 2018: I added a V2 version of the retaining ring with reinforcements around the holes for the shock cords.