So with a few ideas bouncing around in my head, and @nickwheeler doing a CAD design, I had a few routes to go, all based on available materials in my pile.
So I made a rough cardboard replica of the battery.
Nick did this design and a few others.
So I started with a 50mm x 5mm steel strap, bent and cut to bridge the gearbox.
And made up a small frame that the battery tray fitted into.
Welding next.
Looks like a nice fit.
Mocked up with the battery in place, clamp in position.
The feet will only be welded once the rivnuts or captive nuts are drilled and fixed in.
And finally, bracket in place.
Pretty pleased with this outcome.
There may be some gusseting to be done.
Next job will be to create a stand for the airbox to create a raised platform.
Onwards…….. in the words of @pauly