I had been wanting to do something like AM03GT mentioned, but I didn't really want to experiment drilling in my chassis
and I don't even have a drill countersink bit. This solution, though, I believe is the most secure way to hold the battery.
I actually had done something like what Faiz said, and I had some success with it. I had used CA glue to stick it to the chassis, but it came off after 2 weeks with a soft hit in my living room. The only reason why I think it came off is because it was very stiff, and the crashes are less forgiving.
What I like about the strap on the side is that it actually has some flex, meaning if you have a crash it will act as, perhaps, a "side airbag". No need to adjust, drill, or glue anything in the chassis. A simple and effective (at least it seems to be) solution.
I tried it yesterday and it held up fine. But all I really did was drift a little. I will update when I do more testing.