If you don't feel like making your own...I'm running 5000 mah 6 cell nihm, now one on each side, get 1 1/2 +/- hours out of one, has plenty of wheel speed with the 27t motor with 14/86 gears. I save my lipo's for the go fast trucks. I might go ahead and wire it for parallel when I get a little more wrench time available.
with the batteries in the first pic in parallel thats 10000mah total, wow would probably run for ~2 hours! saddle packs work pretty well mounted on the sliders if you are looking for another option.
LoL.... getting close to 1.5+/- hours on one battery now.....Usually get bored with it before a battery ends unless I'm out trailing then I could see running parallel. Then again how hard is it to unplug one battery and plug in another.
So that's my thinking on the subject. Still undecided as there's really no real benefit except run time. Might just make a connector for those days I spend on the trail. Just trying to get a general consensus here to possibly sway me one way or another.