I had trouble thinking of how to mold to contour and stick that chip guard to the roof of the wheel well. So I like the spray on idea. And as JR73 said, it’s easier to touch up later if needed.
For someone looking in the future, it might still be useful to know what material was used and how it was attached.
On my 693R (a '71 S2) there were 2 rectangular pieces made from the same material as the body panels at the back of the arches contact cemented up on the top of the inside of the fender then slathered with a lick of undercoating. They were about 10" long by 6" wide and pretty much centered on the wheel front to back. One on the left, one on the right.
When I redid the body, I took them off...cleaned up the wheel arches, re-cemented them where they came from, coated the entire upper fender with 2 coats of underseal. Yeah, yeah, yeah, added weight, etc.