I've tried all sorts of methods to stop my battery sliding around, especially on micro quads where we are limited to particularly crap battery straps like this. The Leader 120 does ship with velcro pads intended to go on the frame and then every battery you use. These are a pain in the arse (IMO) and they wear out. I've found a great (and inexpensive) alternative - the horribly named Honana HN-CH014 Sticky Gel Cell Pad Anti Slip Phone Pads Kitchen Bathroom House Car Holder
This is a soft gel-like plastic that does not use adhesive tape to stick but it's own tackiness. It leave no residue and wiping with a damp cloth completely "resets" it to it's original tackiness. This is very similar to the pad that Joshua Bardwell discusses here (kyosho zeal vibration absorption) but this kyosho product is approximately $US13 whereas the one mention here is around $US2 at the time of writing. Check out the pics below to see how well it works.
As you can see on the picture at the left, it's easy enough to cut small pieces off as you don't need a lot to do the job and can keep the weight to a minimum. You'll still need a strap or band to hold the battery against it but since I started using this I've had no battery ejections. Not exciting but useful I thought.
Check out this cheap non-slip battery pad for yourself.