[Nick]

Right - I tried it, and the battery fell out again! Although I think I have located the problem.

No matter how tight the straps are and in your soloutions the battery slides horizontally out from the strap towards the motors (to the center) and then the strap isn't covering it, allowing the battery just to come out.

I guess if I make some sort of bar on the other side it will stop them sliding out that way, even a little bubble type rib with duck tape should stop it.

A suggestion in my own experience is make the next straps wider.

Please suggest any more soloutions.