I'd say buy a normal powerbank and a universal attachment. You can even get by with a few velcro cable ties. I bought a connector off aliexpress that's stretchy so it holds really well. Just the arms to attach it to the strap itself aren't that strong, but a couple of extra velcro cable ties around it holds it on nicely. That's with a chunky 20mAh powerbank too. Heavier, but more evenly distributed.
Thank you I'll take a look at that then