I did these two recently. The 2 cell flat is for irishknots of the
Howling Commandos, the 3 cell "triad" configuration is for a local to go
on his Nitron primary and replace a clunker brick-sized 12V nicad pack
of the same capacity that has faded away to nothing.
are Samsung INR18650-20R 2000mAh, which is my current go-to hybrid
cylindrical cell, cheaply available, and having an impeccable service
record and as low an IR as any other cell in that form factor.