Parametric Lens Caps

Need a lens cap? Here's what you do.
Measure the outside diameter of the lens.
Open the freecad file enclosed, and click on Dimensions (it's a spreadsheet).
Fill the dimensions in.
Wait a bit for all the settings to propagate.
Export the "Finished Lens Cap" object as a .obj file (STLs come out with some errors).
Turn off thin wall detection in your slicer (Very important.)
Print.

If the lens you're capping has a hard exterior you might want to increase the Diameter Clearance factor in the spreadsheet. Printing with soft material like TPU may also need a change here.

Note: Requires FreeCAD 0.18.