While it might not make a difference, you don't necessarily need one for a RHD model, just one equipped with a diesel which was a rather rare thing in itself here in the US. You might have to look into ordering one from Australia or buying one on Ebay.
I'm curious, how are you mounting the second battery? I've considered doing something similar, but running my intake like Eric's XJ. I've thought about just picking up a diesel model battery tray, assuming it should just bolt on.
This is what I was talking about...
http://www.spectreperformance.com/i...lter-6-4-diameter-housing-4-inlet-outlet.html