Where are you exhausting to? What is on the other side of your back wall? Is this a stealth grow?
Mine is not a stealth grow. When I got the cabinet it had two four inch 100 cfm muffin fans and filters inside. This cost me about a foot of height. I replaced them with two 200 cfm exhaust fans I mounted outside. I purchased a couple of these ...
Amazon.com
and mounted them using silicon RTV.
I can now change the air ten times per minute and control temp / humidity as well. I also gained some much needed height.