The various 52mm drop-ins DX (& others) sell will fit a Maglite with some slight modding (grinding lip of Maglite head & lip of drop-in module). You can choose to run 18650's or 26650's. Use 3/4" & 1" PVC water pipe or electric conduit for battery spacers (3/4" inside the 1" for 18650 use).
I built a 2D Mag build with the single XM-L DX drop-in running on (2) 18650's. Beamshots here. Not a difficult build if you have a grinder/dremel/files. I then wired direct off the driver to the ground and switch contacts (i.e. I didn't use the switch tower like some do).
This 5x XR-E R2 module might throw more than the single XM-L. Build thread here.
Be careful getting the Mag switch out without damaging it (I destroyed mine). The newer Mags use a T8 torx bit, but the older ones used a 5/64" allen wrench.
-Garry