I've always used at least two straps plus velcro on the tray and battery on all of my balsa planes. Even on my 41" edge, RIP. One just doesn't seem to be enough for me. And I've never had a batt come loose.
I recently crashed my 51 Slick. Broke the motor box clean off at the first former, crushed the cowl, broke off the rudder, and broke a gear in one of the HS5685 on an aileron. I had velcro on the tray an battery plus two straps. The force of the crash locked the velcro between the tray and battery so well that when I went to remove the battery I had to break out what was left of the tray. Just to get the battery out without damaging it.
But I've also had a battery come loose from this system during a crash. I once had a 59 slick do a great 60~70 mph impression of a lawn dart. I think regardless of how much velcro or straps I had on that one. It wouldn't have prevented the battery from impaling itself against the motor screws and going up in flames.
I would highly advise against using that tool box liner to prevent the battery from slipping, go with the velcro. I think if I was using that on my 51 the battery would have impaled itself into the motor screws. And possibly caused more damage.
You should also go over all the motor box, battery tray and all accessible former joints with a little thin CA on any new plane.