I've got a nice bit of ply in the boot already!
In anticipation of going onto non-RFTs I bought a 17inch spacesaver on here and the seller had made up a platform for it to sit on to level up with the step in the boot-floor, Velcro to stick to the carpet and a couple of bolts to secure the wheel.
It's about 18 inches square, so I've probably got more wood than you - mine is just a bit less in yer face!