I would say since it is pre made specifically for a flashlight , no. Not up to me though - just an observation.
Of course a plumbing fitting is also pre made, but it’s not made for a flashlight and there is the difference. Maybe you could find a threaded ‘bung’ or ‘stopper’ from a plumbers/electrical merchant like this sort of thing ? - search google for ‘brass conduit plug’ to see more types
Long answer - I was going to automatically say no but went back and checked the rules from previous years. Justin did allow P60's to be used (but ONLY P60) I guess because they were popular. The P60 was omitted last year. So it wasn't an easy clear answer. It was your own wording that tipped me off - "It would be convenient....". Convenience is using machines or existing lights (both the other categories). Well this comp is named a 'challenge'....
Good clarification - that had been my assumption as well, in regards both to a Convoy S2+ pill and an S2+ triple spacer that I found in my parts bin. Basically my reasoning was, other than the listed items from the first post ("reflectors, drivers, LEDs, O-rings, switches, lenses, clips, sheet metal and tubes etc") if it's made for a flashlight, it's out.
And to expand upon that…. if a factory made flashlight reflector or lens is okay one would assume that includes an optic. ??? But pretty much no other part made for a flashlight.
Unfortunately I have injured my shoulder so no hand tools for me this year but please sign me up for the modification category: https://budgetlightforum.com/t/-/63305