Hi guys... I also used velcro to keep the batt from sliding out the sides, but it didn't hold well when the car was slammed into the wall whilst in forward motion...
So i added a stop brace on the underside of the hold down strap... then i swapped out the velcro for some foam rubber pads which worked just as well...
For some reason the velcro did not like the heat from the battery and often lost it's grip... see the pics below...
Alien