With the onset of gardening season, the home improvement stores are all stocked up with gardening items, once thought to be useless by us RC hobbyists. Not so! Go look for "Plant ties." The stuff I found came in a large 50' ( ~15m) roll for a couple dollars. Hooks on one side, loop on the other. Usually it's green, but I think it comes in a limited variety of colours. It's not really velcro, but it is hook and loop type material that is useful for battery mounting and other strapping applications.
Granted, it doesn't hold exceptionally well, but it is sufficient to use as battery hold downs.
I place matching velcro on my battery and powerpod to prevent the battery from shifting. The plant tie/strapping type stuff is used to hold the battery tight.
In case you were wondering, I just threaded the straps through the bottom of my power pod and added a dollop of hot glue on the inside (bottom strap). This method allows continued full contact between the battery and the power pod. The zip-tie to the inside (top strap) doesn't work that well -- the battery looses contact with the main velcro and swings on the zip-tie.
I've been putting my FT Spitfire through it's paces and have not had any issues with battery movement.
As an added bonus, the stuff is so cheap it can be used to strap together your batteries for transporation.
Or Not. I don't tell you what to do.