Yes, I do have a second battery, but you'll only manage to fit it under the bonnet of a CRD, not a petrol. And even then only after rearranging the existing battery, power distribution block and ECU.
Another alternative is to see what size battery you can squeeze under the front passenger seat.
And yet another option (haven't really thought too hard about this - its really just an observation from some KJ pics from the US) is if you replace the existing air inlet with something like the K&N air-tube (below) you can remove the airbox and possible fit a second battery in there.