I got the backdraft damper over the weekend, so I decided to get it installed right away.
This was the original setup. It worked somewhat okay for something I just threw together.
Here is the new damper. It doesn't seal perfectly, but it will definitely work more reliably than the previous setup.
So, I tore out the old foam damper, and put the new one into place. The foam seal around it held it in place fairly well.
Here it is with the fans blowing. You can see it doesn't open up 100%.
To finish off the install, I packed some open cell foam around the damper. I did this 3 times, packing it in pretty good. I was going to use great stuff, but my can ended up oozing all over, thankfully I did it over a garbage can...
Lastly, I finally got to install this cover which I've had ever since I moved into the house. There is a good 1 inch gap between it and the chimney that allows air to move around from behind it.