Have been thinking about it and think (if they don't exist to purchase, as I would have thought we'd have seen them around by now if they did) that I could mod a set of VN rocker covers by cutting them down to the right height and then getting a correct shaped mating surface welded onto them to suit the tin covers. As long as they cleared the installed height of the rockers I couldn't see why they wouldn't need to be any more than about 15-20mm deep. Hmmmm...