Seems like I'm a couple of months late on this thread but what the hell.
I've just replaced the boot floor on my 95 Disco and have a few comments relating to the issues already raised.
If the vehicle floor has failed the MOT and you plan to have it retested at the same garage, the tester will almost certainly check the repair and for it to pass, the new panel must be continuously seam welded around the the entire perimeter of the new panel. Spot welding in the same way that the original panel was will not pass so for MOT retests, I guess pop riveting is out and marine ply definitely not. That said if you are prepareing the vehicle for the initial MOT test, these techniques may be an option but my advice is weld a replacement part in. I got my replacement floor from Country Rovers in Chard for £65. If you can make your own panel up for less, fine but I wouldn't bother unless totally skint as opposed to nearly completely skint like me. Mine took about 2 hours to drill out the original spot welds, cut out the floor and prepare for welding. About another 2 hours for the welding, painting, seam sealer.
The floor supports on mine were not too bad so I didn't need to replace them. It seems replacement supports can be difficult to get hold of. If so, I would have thought it fairly straight forward to make up something similar.