I had the same idea with the spare wheel area being vacant after I installed my rear bar and wheel carrier and this is what I designed.
I got a 50lt polytank made up to fit the area under the car where the spare wheel was. Got it made up by a fellow in Queensland who also made a water tank for my earlier RR Classic. cost approx $330 delivered to Sydney.
I also got a locally made aluminium bash plate / cradle which uses the existing wheel locking chain plus made mounts to bolt to the chassis. approx $180. I run the filling / breather and outlet via electric pump into the back of the car via the wheel arc, where I mounted to the side of my drop down fridge slide. works really well, please see photos attached.
Attachment 153772 Attachment 153773 Attachment 153774Attachment 153775 To the right of the fridge slide I have the valves.