But then I had some free time this afternoon, and thought I would take a swing at it. Unfortunately what seems simple usually (for me) evolves into something not so simple, and so a project I though would take an hour took multiple hours - and I have still yet to print and test it. I am pretty happy with the sliced model though, and once my Ender3 is freed up tomorrow I will try and print it. If it all checks out, the design will be uploaded to PrusaPrinters and entered in their contest for some filament. It is designed to use a simple pen insert which I pulled out of a BIC pen.
I took some of the inspiration for this from another handy tool I use pretty often, which is a remix of this thing. If you need to sort a lot of nuts and bolts, that is a great tool and makes the process easier than just guessing.
Getting the fit to work with the pen refill and the 4mm bit was probably the most difficult part of this design. I printed a half dozen test parts to get it just right - and I hope it will work similarly for other printers. Getting tolerances right is a big pain with FDM since not everyone's printer/slicer behaves the same, and when dealing with very small close fitting parts it makes it a bit of a balancing act to try to make things work. My printers are not perfect so that adds to the headaches. I try to calibrate things when running test prints like these though, to limit the problems. In any case I hope it will work as well for other folks as it did for me. I am running a final test print now and will post it with some pics probably later today or tomorrow, once those are done.