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