One of these might work

https://www.decathlon.com/products/u...QaAq-REALw_wcB

https://gearwest.com/topeak-versamou...IaAptrEALw_wcB

This one looks the strongest to me

https://dawntodusk.bike/product/bear-hug/

This silicone self fusing tape is excellent to wrap around the tube first to protect the paint. It alo crushes down making for a more secure mounting point. Put this on the tube then mount the external bottle mount on top of it

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

Then once mounted I wrap one of these around the whole battery fixture. The silicone tape protects the paint where this mounts around back, plus adds some friction to keep things from slipping. I carry a couple of these in my touring panniers as well, excellent for securing things to the racks. You can get ones that are stretchy, but these solid ones mount very securely.

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1