You could roll your own using Odyssey batteries. I think (but have not confirmed) that you could fit two Odysseys in the stock location. Did I see someone do similar with Optimas? Not sure if they had aircon though.
I'm sure something could be fangled for the driver's side of the engine bay as well. Given you have to cut/modify the XJ diesel battery tray to make it work you'd be just as well to go to the junk yard and get something close enough or weld up your own tray.
At some point I'll be doing something about batteries. My plan at this point is dual batteries somewhere under the hood, probably one on each side and a third battery (sealed Odyssey) under the rear bench or in the side mounted boonie box where the spare tire currently sits. The third battery will be purely for interior electronics.