I bought a flat bungee cord to hold the battery down. It works pretty well and the length is adjustable from, like, 6" to ridiculously long. This is the exact one I bought:
https://www.amazon.com/Reese-Secure-...+secure+bungee
There wasn't a great place to clip the bungee so I drilled 2 tiny holes and looped a zip tie through it. The bungee hooks on there great.