I believe KISS is the way to look at this.
Option 1- Just lay it in the back, put a strap over it to stop it moving anywhere. Clearly the cheapest quickest and simplest
Option 2- Try and make a wheel mount for the inner of the door if you wanted to, I think it will be a real pain but hey ho!
Option 3- You could spend £200 odd on the raptor mount that goes up the van side and sits on top of the wheel tub but is it really worth it? They also seem to be out of stock and removed from there eBay listings
http://www.raptor4x4.co.uk/raptor-4...carrier-for-land-rover-defender-90110130.html