Any suggestions?

cable ties would be the best bet i suppose?

velcro straps

Seat mounted bottle cage,if the cable is long enough.

A couple of big O-rings?

gaffer tape. Loads of.

Lumicycle have a battery bag with velcro straps which you could probably buy off their website. The straps wrap around the frame tubes. Obviously you would need to check if the battery will fit in the bag and whether your cables will stretch from where ever you can attach the battery. The other considerastion is that velcro straps will cause scratch marks to painted/laqueured/polished frames

Eve Libertine – Member

The other considerastion is that velcro straps will cause scratch marks to painted/laqueured/polished frames

So will mud, dirt and rocks.

Several REAL solutions available, here's a couple:

Strap On Bosses

Saddle Rail Mounted

So will mud, dirt and rocks.

Yes, I often go out for a 2 hour ride with mud, dirt and rocks wrapped tightly around my frame 🙄 . I have a velcro strap battery system and the first thing I did after a few rides experience was to protect the frame from the straps, it was making an utter mess of the frame very quickly. I'm making a valid point and giving advice, from experience, to the OP, what is your point?.

Thanks.
"Strap on Bosses" look good. It's only for a commuter bike so mud / dirt shouldn't be too much of an issue.

i agree about velcro damaging frame finish, i just cut up a bit of old innertube and it works a treat to prevent movement and damage.