Unless you want a 'perfect' replacement then a plate over the boot floor and pop riveted in is fine. Alloy or steel, your choice!
Cut back any loose rust, treat what's left and smother with Schutz.
Use seam sealer or low modulus mastic when you rivet the plate on, job done!
Watch for sitcom pipes if fitted, wiring, fuel tank and pipes etc. don't cut or drill through them!
Depends on just how bad it is. I repaired my boot floor, fabricated new sides and back edge and then a couple of localised patches where needed.
Folding the edge sections was a bit of a challenge but the result was great. The back edge took some effort, I cut a length of steel then cut slots in it, then I clamped the steel to the back of the boot floor panel and tacked it on the "high" spots then using clamps and a hammer bashed the new steel to match the profile of the floor panel, was really pleased with the result. Could e-mail some pics if you like.
go to land rover friendly mot shop and get re tested..its not a monocock..yes i spelt that wrong..i ****sed..but as long as rot 12 inches from body mounts etc it should pass..and that is .