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I cobbled this up in a couple minutes using Tinkercad; I believe it can only export STL files.
As listed on the "Thing Info" tab, I imported (remixed) user gcardinal's F-mount rear lens cap, and user hot_soup_in_space's F-mount body cap in order to make this.

Here is the Tinkercad link for the 40mm adapter if you want to give it a shot. The only difference between all three is the height of a hollow cylinder between the two end caps: https://www.tinkercad.com/things/4w0QuEada8U-40mm-macro-nikon-adapter