I will be installing a Rennai gas direct vent wall heater. The thru the wall vent wants a 3 1/8 hole. Exterior is vinyl siding. It doesn't come with an exterior wall shield and Im not wild about screwing it right to the siding. I could use a vinyl mounting block...but it's vinyl...What else could i use? The direct vent exterior shields Ive used on the gas fireplaces were metal and had a 6 or 8 inch hole, much too big for the 3 inch vent. Any I wrong to be concerned?