I've got a Trance and with a short stem, the battery doesn't fit well underneath. So I'm using the extension cable and putting the battery under the top tube where it meets the seat tube. But the velcro strap doesn't hold it and it flew off on the last ride.

What I could do with is finding some sort of pouch that I could fit the battery in and that has decent fixing straps, ideally straps with buckles. Is anything suitable out there and is anyone else having the same problem?

You should be able to buy spare straps designed for strapping roll mats etc on to rucksacks at any decent outdoors shop, I'd try getting one and cutting it down.

I bought some velcro straps from B&Q

Its dark, its not a fashion parade, just get some duct tape. Sorted.

Raid the IT dept for some velcro cable tidies.
Or cut up a old water bottle and put it in a cage.

Don't Hope make a side to side mount for short stems?
Failing that, I've cable tied mine under the seat before.

I ran mine in a seatpack at SITS running the long cable down the top tube.