Use a piece of 1mm styrene sheet cut to 12x9mm and then remove the corners (see the measurements shown in the image on the previous page).
Once glued on to the the kit light housing, chamfer the edges with a scalpel and then file and sand to acheive a rounded housing that looks a bit more automotive than the unfinished hole the kit comes with. I know the 1:1 lights have a flat back, but this looks cool.
Worth 10 minutes of anyone's time!