I use a similar method with an inner tube cut to size. as far as fitting I just wiggle it in. 1 goes around the battery holders, the other is used to keep the connector tight against the holder. Since I have strated to use both methos I have never had a disconnected power source.
After I ride home which may take an 1 1/2 - 2 1/2 hours based on if I do some training I take the pack out and and pop in 4 recharged batteries so that I do not have to worry about riding and forgetting while on the road. I additionally leave 4 Eneloops inside my saddle bag just in case I'm out longer or one battery goes bad.