Basically, you remove the entire mechanism from the car, make a note (or take a pic) of where the wheel hanger is in relation to the mounting, and then cut the winder mechanism off the three-bolt mounting plate, which you keep.
(The winder mechanism can then be sacrificed to whatever god deals with mechanical design)
Now you need a length of studding, I'd use 12mm, get it welded to the mounting plate in the correct position (as determined above), it obviously needs to be long enough to pass through whatever wheel you're using as a spare, plus 60mm or so for the retaining nut and washer(s).
The wheel needs to be hung with the outer face downwards, otherwise the face of the wheel will bear on the mounting, scratching it to bu**ery in no time.
It could be mounted the other way up, but you'd need to have a spacer machined up to fit snugly in the centre bore, to keep the wheel from touching the mounting.
You'll need a large washer to go between the wheel and securing nut, it needs to be larger than the centre bore on the wheel, obviously.
Stick a bit of rubber between the washer and wheel to prevent scratching, if it's an Alloy wheel.
If welding's not an option, you may be able to drill the plate and attach the studding with a couple of lock-nuts either side.
I'n not sure, as I've not done it myself, I simply keep a full-size spare and small trolley-jack in the 'boot'.
Will get round to it one day I guess . . .