My intent is to build this to run from a single 3.7v cell. I’ve looked at 53mm drop ins and would love to go that route but could only find one that claimed to run from 3-8.4v. It wasn’t impressive, any suggestions?
Copper slug in the top of the tube, FET driver, XP-L HI with 45mm TIR optic. Efest Purple IMR26650 4200mAh cell. You could actually use an XM-L2 U4 1C and get around 1800 lumens with the stock reflector modified slightly.
Fairly easy, punchy high output light with decent run time.
Edit: Ok, that was a short answer. Use aluminum or copper to make a pretty huge heat sink that fits into the top of the battery tube. Remove the switch assembly by pinching out the rubber clicky cover and loosening the allen screw inside the switch that grounds to the inner tube, then the entire assembly will slip out the tail end.
Cut off the pillar that the existing bulb mounts in, make it flush on top of the switch assembly. You’ll need that space for the heat sink. Cut off the tubular column at the bottom of the reflector, try to keep the outer ring even with the aperture of the reflector, but with no tube left, only the parabolic style left. This will still be hard mounted in the head of the light and you’ll still be able to screw the head up and down to fine tune focus.
Grab a FET driver from Richard at mountainelectronics.com. He can configure it to suit your needs. You can get fancy and cut a pocket in the base of the heat sink for the driver, or simply glue the driver onto the bottom of the heat sink with JB Weld. Drill a hole through the 1.5” heat sink for your emitter wires. The 1.5” diameter stock needs to be sanded down slightly for a press fit inside the Mag tube. Also sand the anodizing out of the top of the tube so the heat sink is grounded to the aluminum of the tube. This makes it easier to attach a ground wire from the switch assembly to the base of the heat sink with a tapped hole and a machine screw. Power lead to the driver from the switch, emitter leads through the sink to the emitter on top. Figure out how high the emitter can be and still meet the reflector when the head is twisted all the way on, or nearly so, and pound that heat sink into the tube (after connecting the driver to the switch… don’t forget to anchor the switch with that internal alan screw before replacing the rubber switch cover)
BAM! Mega Mag. (cut a spacer for single cell use in the tube, or chop the tube if you’re really feeling boisterous!)
I hope you dont have to sack your assistant like I had to sack mine. 
Thanks. I was afraid of that answer. Guess I’ll need to need to ruin some electronic components while I re-learn how to solder.
Ok. Wow thanks Dale. I’ll be converting to a tail switch but great info. It will be chopped. And MRsDNF my assistant is safe, for now… It’s only been 4 years.
Flux is your friend…
Built a 3D Mag a while back using three 26700, MT-G2, HX-1175b1 driver. A big XHP would be cool. Used the OP reflector below and it is heavy for good mass to add to cooling.
http://www.kaidomain.com/product/details.S009717