A simple, friction-fitting cover ring for the knurled viewfinder eyepiece on a FED-2 (ФЕД-2.)  The knurling had a habit of scratching my spectacle lenses, but this discreet accessory eliminates that problem and doesn't interfere with the leather carry case.  It also seems to improve the viewfinder contrast a little.  May also be a benefit for users without spectacles.

It's made thin to stay as close as possible to the eyepiece glass, but has just enough thickness to not be flimsy.

Known to fit FED-2 models with an eyepiece fitting diameter of 12 mm. I don't know if it will fit other models.

Model Choice and Fitting

There are three different models here, all the same but apart from slightly differing internal diameters.  The correct fit is hard to predict for other filaments and hardware, so I recommend printing all three and testing.

The right part should push on all the way and touch the camera's body shell.  The fit should be tight enough so that it's very difficult to remove with fingers, but not so tight that there is some visible distortion to the part.

Printing

Use black for best results.  PETG is good. PLA is maybe a bit hard, but should be OK.  TPU seems like it aught to be among the better choices, but I found it difficult to get a good fit and stay on tightly.

A smooth print surface is very preferable.

0.2mm layers / 0.4mm nozzle or smaller is fine. Other normal, recommended settings.