i built a battery box for the uproar which is tight after flights. using 2 2200 20c 4cells in parralel. the reason i say this is i also put a strap under one of the batteries to pull up on thus not pulling on the wires.
this idea will work for most battery locations where its tough to get the battery out. the strap i use is duct tape folded over on its self so it weighs nothing and is very thin.
strap not shown in picture but just wanted to show the tight fit for the batts and they are the perfect weight so cg is met without lead added anywhere.