Thanks for the recommendations, guys! The NL drop-in is one that I was definitely considering. I imagine that I'll end up with more than one drop-in so I can play around with some different options. I just picked up some KeepPower 2500mAh 16650 cells for my MDC and I assume those will work in the 6P as well as with the Malkoff drop-ins?
Those'll probably work.
Mr Malkoff built his modules for use in a wide range of voltages. You can see that when at malkoff devices reading details of given M61... iirc near the bottom of the description.