I can't help thinking a visit to the scrapyard is in order.
Lots of cars have these type of arch liners, and something should be found to fit, or that can be trimmed to fit. They're made of softish ABS(?) plastic that would adjust easily.
I think the key dimensions are the radius of the wheel arch lip, wheel and tyre diameter (which will relate to the arch lip anyway) and the distance from arch lip to inner wing. Look for something about the same, or from a slightly bigger car. You could even cut a cardboard template and take it with you, to offer up to cars in the scrapper. Hopefully it wouldn't be raining that day
The highwood ones look very nice, but I really couldn't justify the cost.
I'd be prepared to have a go myself with a sheet of Ally as well. Cardboard template to make the mistakes, and copy from it. You can buy rubber edging strips from places like Car Builder Solutions, which would prevent chafing the paint/underseal at the edges. I don't really see the need to do it in an expensive material like ally though.
This is my 'thinking out loud' view, and if anyone thinks I've got it wrong, I'd be happy to learn something from that.