This is an adapter to go from the old school Canon FL/FD lens mount type to the newer canon EF mount.
It has not been tested for focusing distance, I didn't measure flange distance or anything... I will revise if I have issues

Update 4-28-22 I have had some comments about if this design is functioning. I never totally finished the design, but if I remember correctly, it works for most of the focus range and has trouble focusing to infinity. I believe the tolerance for the actual mounts was decent and it fit both the lens and body fine, given a few minutes of filing to clean it up. I would warn about using it on any body that isn’t basically a throwaway because the sharp EF mount on the body could create plastic shavings that could get stuck to the sensor via static or just get into the body in general. If you have a speed booster or anything to put between the plastic mount and the body I would recommend doing that.
Also, I have had a few people ask for the CAD files. I made this design years ago and I did it in Tinkercad of all things, so unless you want the tinkercad file, no, I don’t have the CAD files. If you wanted to play around with the flange distance, I would recommend importing the STL into your CAD software, do a plane cut on one end of the adapter with the place butting up against one of the adapter walls, and extruding the middle section however much you want. I’m sure someone knows a better way to do it though so don’t let me tell you what to do.