I've created a pinhole shutter, primarily for my own use on some experimental cameras. When I put the design together, I made it compatible with the Goodman One bellows. Assembly uses some M3 and M2 screws I had available, as well as some 2mm x 1mm disk magnets I got specifically for the purpose of giving the shutter a snappy action and holding it in place. The assembly uses a 20mm pinhole disk as is commonly available on the eBay.
Here are the files: https://www.thingiverse.com/thing:3555844