I'm using the IP 1600mah 2S 40C pack. I think it's intended for 1/18th cars, but it works fine in the RM-01.
To hold the battery in place I screwed together some old plastic servo posts and right angle parts from an M03 C- parts tree. It required 2 holes to be drilled and countersunk in the chassis, but it seems to work pretty well. I can take the pack in and out quickly without using any glass tape which suits me
