Back Door spare wheel carriers

I have always used the bonnet type spare wheel carrier, but am now wanting to utilise the back door type. Recently obtained a SIII or even a Defender one, incomplete. Obviously the whole weight of the wheel isn't hanging on the alloy skin of the back door; I suspect that it's some sort of sandwich ass'y with a plate on the inside as well - this is missing but can be fabricated.
Any fitting details anywhere?

What sort have you got ? as usually the plate with the 3 bolts on goes thru the door, and a plate then goes over that on the outside IIRC