Phew, so it's not a total bust then. As it happens, I was hoping to extend it a lot further, but after some trimming and folding this was what I ended up with. It is at least quite solid. Today I sprayed it with a little stonechip paint to (try to) match the textured plastic on the rest of the car:
Rear visibility is still ok, just about. The issue of lift hadn't really occurred to me. It did feel a little more tail-happy into corners when driving at speed - I had assumed that was the weight of all the beer I had loaded in the boot for midsummer.