Step 3: Installing the fan - because removing the AC left an odd sized hole, I needed to use metal snips to cut away about 1/4" of sheet metal followed by some filing of the sharp edges so the fan would fit. This still left two gaps on the left and right of the fan, so I went to a local metal supply and got them to cut two aluminum plates to cover the gaps. I used a layer of weather stripping, a fat bead of caulk, and self tapping screws to fasten and seal it to the roof. This proved unsuccessful at first, as my leak test failed a number of times. I had to apply larger beads of caulk, and also coated the plates with a few layers of white plasti-dip before it became leak free.