This is a simple body cap for a pinhole to mount on M42 threads.

The pinhole is mounted close to the flange of the M42 thread. The design places it 0.2mm behind to take into account a fit that is likely to be less than theoretically tight; it should be about right in practice.

You could use this mount to make a pinhole camera of known focal length with extension tubes, or variable focal length with a combination of helicoids and extension tubes.

I printed this on an Ender-2 Pro with Creality filament, using PrusaSlicer defaults. I used 0.16mm layer height, with variable layer heights enabled (default settings).

Print the main body face down, and use supports (I used snug supports). You might want to increase the support X/Y distance a little to make it easier to remove.

This design uses the same 36mm mount discs as my wide-angle pinhole body cap for Sony mirrorless cameras so if you've printed that, you don't need to print them again. 

For the pinhole material I use a piece of kitchen foil, smoothed on a solid surface. Once smooth, hold the foil against the solid surface and pierce the foil with a needle (a nearby nozzle maintenance needle, perhaps!)

This way you get to make a hole with just the very tip of the needle. 

Then clip the foil between the retaining ring and the mount, pressing the retaining ring firmly in, and press the mount into the main body. This holds the foil nice and flat, and might be important for supporting small diameter pre-cut pinhole sheets.

You may need to file or sand the retaining ring and/or mount so they can be pressed into their respective spots.