The metal cover should do great, but you want the base of the antenna a
little above that, in other words, don't ground mount the antennae. You
could mount it to a supporting post and that should work. Ideally, the
antenna should be in the center, just like mounting a mobile antenna on
the roof of a car, but even on one edge would work for you. You'll
actually have a weak or flat side to the radiation pattern where you don't
have a ground. For example, if you put a gutter mount on the driver side
of your car, the poorest radiation would be toward the driver side. The
cover is a good idea if you can get it mounted.
73s
Butch Bussen
wa0vjr