I think the only viable option is to leave the spare, so I'm going to plan with that in mind. I have been contemplating the Titan rear drawers from 4WD Supa Centre as they measure 1000mm W x 900mm D, for only $365. I had them in the Hilux before and while not perfect, it's hard to beat it for the price.
Being that they are built with a steel frame, it should be easy to create a cross beam along the back seat where the child/cargo restraints are and have the drawers hinge off that. Only issues I can see are the weight, at 71kg, and the drawers aren't so easy to remove.
Another solution I'm thinking of is building my own drawers out of particle board/MDF/Plywood (a local business discards a lot of this stuff) so it will depend on whats available at the time. But at least it's all free
Or, I'm going to reuse the aluminium drawers I created when I had a JK. I haven't got access to them at the moment so I can't check the measurements to see how well they fit. Although, being made from the aluminium qubelok stuff, I'm going to have to reinforce it a fair amount and/or create a false floor so I can add the hinges to lift it up. With all this extra reinforcing and given the drawers are actually 3mm thick aluminium, I probably won't be saving too much in terms of weight over the Titan drawers.
The JK drawers originally: (have since been hacked to remove the big drawer and have the 2 smaller drawers side by side)