Alright, I've got the battery tray/strap/spur guard/whatever done.
After about 2 hours of making and remaking and refining card templates, I finally traced a pattern onto some 0.5mm alu sheet and cut it out with the dremel wheel, then bent it up and drilled a few holes. It's held down by the posts and also acts as another diff brace.
With the battery in place:
Then I cut up a bit of 3mm angle and drilled a couple more holes, then put a bend or two in:
All the extra alloy on the battery side helps balance it a bit, it was a little motor-side heavy before (only very slight though). Haven't checked the balance yet with the new bits, but I'd say its evened it out a but.