Many monoculars and binoculars either don't come with lens caps or have caps that separate, making them easy to lose. This cap has a ring that goes over the eyepiece (or objective lens) that allows the cap to flip down without getting lost.

As shown here the ring inner diameter is 44mm and the cap 44.5mm, but you can modify the dimensions by “tinkering” the model at Monocular Cap (tinkercad.com)

I printed this in a generic-brand TPU. The Engineering plate works well for this - keep in mind that TPU sticks VERY well to the plate, so I recommend against using a sticker-based plate.