Im running a 1D Maglite with a 2-3 cell Malkoff dropin.
I can use either a AW 18650 cell with a homemade sleeve made
out of plastic packaging wrap or using a Kaidomain D size Li-ion cell.
If you use the Kia D cell you need a magnetic spacer on top of the cell
to make electrical contact with the Mag.
Also get one of their replacement "gold" battery springs, these make better
contact with the bottom of the 18650 cell.
If you order the cell from Kaidomain, while your at it, grab one of their
MOP reflectors the you'll be laughing