The P60 drop-in is faulty by design, as far as heat transferral is concerned. Crikey, all those bloody drop-ins are designed very poorly!
Have you ever taken apart one of those? No offense, as I do know a lot of you probably have. That brass pill which screws into the aluminium reflector, well, to be honest - I couldn’t think of a design more flimsily. Subsequently, I’d add as much copper to the pill as only possible; press-fit, then add a thin line of solder, just to make sure.
Next comes the design itself: Heat, for that matter, is mostly drawn off by the wrapped aluminium reflector to the head of the torch. Which, in turn, is not the part where the heat actually builds up - that would be the junction point between the emitter star and the pill, or more comprehensively, the pill by itself.
Now, you’d still have to overcome that terribly bad barrier of threads between the pill and the reflector, to effectively draw off the excess heat.
Think about that, sometime. Standard LED P60s are awful to heatsink properly.
Oh, just to add one more thought - a thought, really, an idea, nothing proven by myself, so don’t club me to death if I should be wrong:
Copper is only as good as the alloy it will come as. Meaning: Pure (chemically pure!) copper = excellent, use it if you can get your hands on it!; industrial-grade-copper (a.k.a. “what-the-machine-shops-with-all-their-fancy-CNC-lathes-usually-have-piled-as-bar-stock”) = save your effort and take your commercially available aluminium bar stock, as it can be machined much more easily, and transfers heat at the same rate as a generic copper alloy.
please take this into consideration as well.
Oh, on a side note: Of course!!! I do use copper with a pill rather than aluminium - just for the sole factor that soldering copper to brass is SOO incredibly easy, compared to soldering aluminium to brass.
your best bet would be a conically shaped liner bushing, and both the pill and reflector re-worked on a lathe to match that conical shape of the bushing, then hand-lap, then press-fit everything together. Sounds like a lot of work? It surely is, at least for me. Angles of any kind I have always found terrible, be it on 3ds max, AutoCAD, Nemetschek Allplan CAD and even by hand.