you want a lot more than a solid plate bolted to 2mm boot floor steel !
has to go direct to chassis through suitable gauge steel ,should be fairly simple and require bumper either cutting or replacing with a suitable mod ,
if mounted low enough you could just have a hatch in the boot for maintenence
What Ginge says .. either a custom rear bumper, custom rear cross-member and cutout bumper, or tray between chassis rails. VEry little call for them, but it was something I considered for a while and looked into.
My favourite idea was a standard winch tray cut to fit between the chassis rails, and a custom bumper made to suit that also reduced overhang ... you _really_ don't want to add to the overhang of a Disco, especially if you're putting over 40 kilos of weight at the very back!
Something like this but with a winch tray .. and probably without all the fancy bits, though a swingaway wheel carrier as part of it would have been a nice touch ..
What about front and rear receiver hitch things and one winch on Anderson plug.
You will only ever need to use it at one end at a time.
I am sure someone like press brake could easily make something.