Ok, my bad, I'm just not used to that
Up until now I've only had Short Course / 1:8 offroad cars, and securing the battery is pretty important there because a battery coming loose can rip the innards of the car apart pretty bad. (And jumps can dislocate the battery pretty easily if you don't make sure it's absolutely 100% secured in place)

I've test-fitted my square lipo and it's basically loose in the chassis - there's slack space vertically, and it can move freely sideways. Basically, I don't have a clear idea of how I'd fit it securely.
Would you mind posting some close-ups from both sides showing how your battery placement works ?

Edit:
I did find this old post from someone using a square lipo in a "v1" M05 but that seems to be a modded version of the "round" battery setup : http://www.rctech.net/forum/10834511-post21.html