For posterity, I figured I'd round this out with how I do this. It's crude and not perfect but it works.
Take DX SKU 14599, which comes in a 2-pack (good because there's a good chance you'll screw it up the first time) and either patiently file down the base of the reflector (where the module would screw into) or the quicker approach is to use a large pipe cutter and cut off about half of the threaded portion. The reason we do this is because regular P60 dropins you find are a more course thread than the 14599. The reflector comes with a threaded pill if you wanted to solder your own LED and driver board, but I like using completed parts. Once the base is cut down and dressed (i use a wire wheel on a bench grinder) you can force thread the pill into the base:
For a more flush fitting bezel, you can spend about 30 minutes with a flat file, rubbing the top lip of the bezel until you remove about half of the material. You dont HAVE to do this, and you risk messing up the coating on the portion of the reflector that actually matters.
The result is a relatively robust LED dropin that fits inside the X3 head. The later X3s are slightly different than the ones I've worked on, and they might require removing the lip of the reflector entirely. They also have thicker glass lenses and might need a thinner one (DX SKU 10643) to allow the bezel to screw down all the way.
