I think the easiest solution is to cut another disk of copper, lap that, and drop it into the cap and solder it. Then, drill it for your wires( being careful to dremel the burrs around the holes.
Good idea.
I am not sure that the 4-6% gain in thermal resistance at the interface is worth the effort I put in to that as both lights run cool enough. But for those wanting to have the copper pipe cap work at its very best, I add the following:
I solved the issue of polishing inside the caps by just using the caps reversed so I could lap their bottoms. One was made into a pill like in flashlights held in a copper liner by a ridge and valley keyway system shown here at the top of the post:
https://www.candlepowerforums.com/posts/3217270&postcount=27
The other was a pipe cap fit into the back of the body shown in the botttom of the same post.
I melted a pool of silver solder in the back of the last version to increase the radial thermal path to the light body and the solder obligingly made a negative miniscus thinnest in the center. Nice surprise. (The pill has a thick copper insert, (heavy and overkill), but it slows warm up.)
I used a hot plate to melt the solder in the cap. Worked like a charm. So I might suggest that the hole in the cap be made before the polished copper plate is soldered so excess solder can exit and not well up at the edges. Put a spiral of solder wire under the plateand place the plate/solder/cap on a hot plate. With the hole between the heating element courses. Any leaked solder will lift off the pan underneath when you are done.
If you need to enlarge the copper cap to get a good interference fit in the Marwi Bullet housing, I'd add the copper wire and solder wire around the outside of the cap to do this at the same time in one step, (if you aren't goin to use the flower petal method to expand the cap as used to shrink the backside of both my heat sinks). Drill the hole through the new added bottom from its side to reduce burrs. Then file and sand the enlarged cap cicumverence to get a nice snug fit. Some Arctic Alumina, and all will be well.
Or for the Marwi Bullet light, just cut a 5 mm thick slab of aluminum bar a bit bigger than the ID about 32 mm for a slug. Drill a hole for wiring and any screws then sand to fit and polish as much as you want. Arctic Epoxy it at the depth you need it up to 20 mm. Done.