Chris,
What kind of heater are you talking about?  From the context, it sounds
like you are talking about the free standing kind with the gas hose that
goes to the wall.  Is this correct?
 
If so, some of those "breath" (or are cooled) through the bottom;
blocking off that area might cause a problem.  If it's the kind I am
thinking of, the only real solution I can see is to elevate it about 18".
Other than that, the only other thing that comes to mind is a large wire
mesh cage to place over it.
 
HTH,
Gary
[Posted in FML issue 4301]