Hi Stewart,
I fly the 36" Tomboy and just use a standard AAA NiMH 4.8v 800mAH pack from Overlander. The weight diffrence is only about 11g and if you buy the square pack it will sit between F1 and F2 which puts it ahead of the C of G which is the best place for a little extra weight!
This gives me piece of mind that I'm not going to run out of battery!
Anyway, enjoy your Tomboy
Bob