Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4931832.
This idea isn't new, but I did create the design from scratch to suite the M3 nuts and bolts that I have available and the bearing. The main idea is that a simple switch detects the presence or absence of the filament, but because of the filament path too much tension on the filament (when there is a jam) will also pull it away from the switch and hence trigger a run-out.
The spring is just from a simple pen and might not even be required if you tension the bolt holding the arm sufficiently. After the jam detection goes off, you might have to manually press the arm manually back in place, but you anyway need to resolve the jam in that case. This design is working very well on my Ender 3 Pro (older iteration) and my Sapphire Plus (the iteration included here).
The printed bushing is used to fit the 608zz around the 20mm M3 bolt going through the center of the arm. You add a standard end-stop button (not a full end-stop module, just the switch - one with a metal bar that's bent in half a circle at the end) between back body and front (and if you want to use a spring, also an M3 nut where the bolt holding the spring would go) and then bolt those tightly together with some 10mm M3 bolts - except for where the arm should get attached. There you first put the arm on and then tighten everything together with a 20mm M3. Depending on the tension wanted, you can add a spring and M3 bolt near the front of the arm. Add a washer between spring and arm to avoid it goes in the slit of the arm. The arm doesn't have to move up too easily, but should be able to open up when the filament tension becomes too high due to a jam.
Then finally you can add 2 PTFE tube couplers with 6mm threads.
No supports should be needed and printing should be fairly easy.