Not a big fan of Velcro, since it can be hard to release if the battery is in a confined area like a boat hull. I think I may have stumbled onto the perfect strap system!
I bought for $2 (aliexpress.com) a silicone watch strap. It uses a stainless steel, fold-over type clasp and the silicone stretches and grips the battery perfectly! Easy to flip the clasp open and closed for really easy battery removals. You'll have to figure out how to mount the open ends, but there are lots of ways to do this. A zip tie works perfectly in my case.
Thought I'd share.....
