A lens cap holder I designed for my wife, holds 52mm, 58mm, and 62mm lens caps. Each size has the correct threading just like the lens itself, so they fit perfectly and rigidly so they're going nowhere. Designed to be slipped on to a camera strap or bag/backpack strap no wider that 25mm or 1inch. There is a small cutout on each strap guide thing so it can easily be attached to the strap without having to thread it on from an end.

Highly recommend printing at 0.1mm layers so the threads print properly. 100% infill for strength. No supports needed. I printed in PLA and it's been absolutely solid, but use PETG if you're heavy handed.