The original Thing by bookmake was nearly a perfect fit for Maplin's "Minature Microswitch with Roller" (Maplin product code N97AQ), except the gap for the lever arm and roller wasn't quite right - didn't extend far enough towards the left.
So, I took their STL in to Fusion360, cut away a bunch of the "out" side of the filament guide path. A test print revealed that the part was also a little too small along the Y-axis such that the switch was being clicked just by putting the microswitch in to position, so I stretched out the space between switch-holder and filament path by 1.5mm to solve that. (The part is not just rescaled in whole - I cut it in a couple of places and stretched just the necessary area.)
Unfortunately I wasn't able to get Fusion to behave to resize the lid in the same way, so I just trimmed it back. It still works (the lid helps hold the switch in place), so I've called it "good enough".
I have added a further variant file, with an open slot in the base box for wiring (rather than a hole) to give the option of not having soldered wires "tie" the box to your wiring. The same lid still works and nicely keeps the wires in place, it's just easier to work with.
I will be wiring the switch up to my Raspberry Pi running OctoPrint and using the filament sensor plugin to get prints to automatically pause at run-out.
The original snapped together really nicely and I think I've managed to not ruin that part of the design.