I used silicone sealent on the back of the diamond plate. Once i screwed it to the window frame (I used self tapping screws), I ran a bead of waterproof sealent across the top, bottom, and around the edges. I used a waterproof sealent I got from the pet store, it's used for fish tanks, they have clear and black. I've been through some major rain storms since, and havent had any leaking problems since. Total cost was about 20 bucks.
Once I screwed it in, the places around the pivot area were nice and flush. Like I said, I have had zero leaking since i did this.