30+ years ago when I first started venturing into France in a Triumph, I used to buy a little bottle of "paint on yellow" that was readitly available at the time. Worked a treat and cleaned off easily with white spirit up my return
LINK!
A quick google would seem to imply that it's still readily available...
I assume you want these covers for aesthetic reasons rather than compliance? If for compliance, why not just pick up a set of LHD 5 1/4" Cibies or similar?