Renault actually do a piece of steel that fits perfectly in place of where the spare wheel well used to be. So you can just cut the wheel
well out and weld it in place, seam seal it and paint it, done.
I'll get the part number for you later on if you're interested.
You shouldn't just weld in a flat sheet of steel as it could reduce the impact strength of the rear end.
The plate you get from renault has been pressed into shape so it is almost corrugated (unsure on spelling) so it is stronger.
I personally think the 172 exhaust is better, the cat to centre pipe join on the 182 is pants!