I found a clip-on L plate via Amazon, which satisfies the first five points but not the sixth (obstructing a parking sensor). We can work around the parking sensor being obstructed by turning them off, but that's a nuisance, and in any case I want the parking sensors working.

The clip-on L plate works by having a spring-loaded number plate clip (the type that you can fit to trailers etc) used "in reverse" as it were. The clip is bolted to the L plate and then that clips on to the car's number plate from the front.

I am still wondering about a complete solution. To get the L plate clear of the parking sensor AND the headlight, it would need to be considerably outboard of the number plate. Clipping to one end of the number plate isn't going to cut it because there would be too much leverage.

I pondered about the possibility of a big bit of perspex that clips on to the number plate at both ends, with L plates at either end, and holes for the number plate and parking sensors. Having clips at either end of the number plate would provide for good leverage to support something outboard of the number plate. One end of it would look like this (with apologies to character-cell artists):

+–––––––––––––––––––––––––––+

| \ | +–––––––––––––––––––––––––––––' | ###### ' | ###### +–––––––––––––––––––' | ###### * | ' | ###### | \ / \ / ‾‾ ' | ###### +–––––+ | \ / \ / | | ' | ###### | | | X X ––+ ' | ################ | O | | / \ / \ | ' | ################ | | | / \ / \ __ ' | ################ +–––––+ * | ' | +–––––––––––––––––––' | ' +–––––––––––––––––––––––––––––––––––––––––––––––––––––––––––' Hole for parking sensor ^{^} Parking sensor ^ Number plate clip fixing bolts-^ Hole for number plate