said by boogi man:I'd use aluminum angle over galvanized because when you drill the holes the rust will start there. so Al angle and a few stainless screws and you're good to go.
I agree with the boogi man
but would clarify to use two pieces of aluminum (or stainless steel) 90 degree angle (3/4" or at most 1") mounted exactly where you have the two black lines in the picture. The reason for doing two narrow angles instead of a wide blade is to minimize the blocking of vent area. Even 1/2" right angle might be sufficient but makes it harder to access the hex nuts (3/4" allows you to leave enough space for regular thickwall sockets).
As an example look at Stanley Hardware P/N: 342154 which is being sold with motor mounds as a listed application.
Drill 2 holes in the center of each angle bracket to mount the motor using the existing bolts and nuts plus an additional 1 or 2 holes at each end for sheetmetal screws.
