I was musing to myself the other day...
That if you laid down the stock bat box, it would just about fit between the clutch cable and the frame with the airbox removed.
You'd need a plate to mount it on because the back side is mostly open and it needs to be elevated a bit to clear the frame.
You could do that with a lithium ion battery cause they are short or can be laid on their side too.
This would give you plenty of room to mount the TDI, rectifier and starter relay with room left over.

I'm changing up the beast and taking the bat box from down under to someplace more accessible and shortening the wiring up.