Richard,
that looks smart. The cable bit is scary. Will you try driving with cardboard version in place or bouncing the car up & down to check for witness marks?
The wider shelves would be useful.
Thanks John. Spent enough time on my back under car to be fairly confident dimensions ok although tweaked a few to be sure. Will probably try a local sheet metal fabricater in Norwich and if quote is ok go ahead and have it made in 1.5 or 2mm aluminium.
Didn't have camera with me when cable issue came to light but found this pic of my car whilst in tin shop.

All looks innocent, but add spare wheel, hood, full tank and two occupants and those coils are going to compress. Although you can't see from this angle the protruding bolt is a tad closer than thickness of loom to wooden inner wing.