There are loads of ways to do this. I’m part way through a build at the mo. I took the simple approach as this will be a light for fun, not for depending on.
I’m using this drop in:
http://www.fasttech.com/products/0/10000504/1051801-empty-drop-in-module-waluminum-reflector-brass-ba
Although I got mine from DX, but FastTech is cheaper.
This drop in fits straight into an incan D cell Maglite head. If you don’t want to mess about it fits well enough to fit the bezel and screw it down as far as the o-ring. Personally I’m happy with this. If you want it to fit perfectly so the bezel will screw down those last few mm, then you’ll need to trim the drop in and/or head a little.
To refit the head you’ll need to remove the switch from the body and trim the old post off where the bulb would go. Lots of vids on YouTube on how to remove the switch. Although it’s pretty easy, just pull the rubber cover off and insert a allen key and unscrew the locking screw, it should then slide out.
Use hack saw or something to cut the post off, you’ll want the metal contact tabs however.
To keep my build simply I was going to direct drive mine, as honestly this isn’t the sort of light I’d want or need a low mode on. And as I said, it’d be for fun, so I have no expectations about needing to use it for prolonged use.
Heat is likely to be an issue, if I find it builds up too quickly, I would add some additional heat sinking material to the drop in and between it and the switch. Probably a chunk of aluminium bar.
Wiring is then easy, just solder two wires of suitable gauge to the switch, feed them up through the drop in and solder to the LED star.
I was going to use a noctigon mounted MT-G2 from:
http://intl-outdoor.com/led-mtg2-c-107_138.html
The only reason I’ve not finished this yet, is I haven’t wanted to spend out on the LED itself. The MT-G2’s are pricey compared to other LED’s.
Power wise, I too was going to run it on 2x26650. I’ve already had the Mag running with these before modding the switch. Very easy, just cut down and bend the spring in the tailcap. The 26650’s are narrower than D cells, so can rattle a little. When I’m finished I was going to make a battery tube out of some PVC pipe from a plumbing store. But functionally there is no need.