The most important custom part is the led shelf/heatsink. For a MT-G2 build it is particularly difficult because you have to work it in the body and use a “tower” style led holder.
I went with the (easier) multi led path and used 4x XM-L2 with individual carclo optics in my 2D mag… check here .
The Ledil rocket (50mm triple) looks cleaner than separate optics. A chunk of aluminum bar stock will do but it’s easier to have someone (either here or near your home) make one for you. The H22 sink is for single led application. Personally I would just make my own out of copper pipe. Even if I didn’t have a torch this kind of soldering can be done on the stove in a pie tin.
It should drop straight into a stock Mag head with no mods. The bezel will fit, but will leave an O-ring sized gap. If you want you can trim the reflector and/or head to make it fit perfect, but there is no need apart from visual looks.
Suspect this is the cheapest way to do it. Should be f’ing bright, but will get hot and with no modes, maybe nothing more than a play light. But will work.
Depending how hard you want to push the LED, there are various driver options, all would be easy to accommodate with this setup.
Any other MT-G2 build will be more custom and expensive IMO.
Same sort of thing would work for any other emitter, although you won’t be able direct drive the others off of 2xLi-ion due to the voltage, only the MT-G2.
As for the 4D. Same sort of options, but you could run many different battery setups. But it’s still going to be a big old tank of a light.
Running AA in series in 3xAA holders might be interesting, but that’s 12 AA’s in total. which just sounds like a lot and a lot to keep charged.
If you just want to make it a usable light, get a simple drop in like these: