Timex- I put the Ultra Breeze in my unit this summer over the vent with the fan. I liked it so much I'm going to put one over the vent above the bed next summer.
Since you have a Create-A-Breeze vent already, you are exactly correct on the installation. The only hard part (and it wasn't hard) is to find the 4 screws you need to remove. You will peel off the Dicor and find the first screw. Once you find the first screw, finding the other 3 is quick work. Take out the 4 old screws, mount the brackets with the new screws and then cover everything with Dicor self leveling. The Dicor has to be flat enough to mount the cover to the brackets, but it wasn't a problem. Attach the cover and you are done.
Windrider is far more experienced than I am, but this vent cover does not have "L" brackets. It has flat brackets. You remove the 4 old screws. You do not have to drill anything.
I kept my vent open most of the summer and never had any rain problems. Worked well.