If you are having trouble with forming a uniform RTV gasket on the Cooler Box door, you might rather put a good release agent on the door and form the gasket on the perimeter of the firewall heat inlet. Keep the door door closed with a small bungee cord for more than 24 hours (until the RTV is good and dry) and then you will have a form-fitting gasket. Take a sharp Xacto knife and cut out any holes in the firewall that might have been closed. It worked for me.
JT
JT