Captain o'th Black Pearl
Yeah, that sounds like it would work, making the flap so it would have the clearance to open/close inside the T-fitting might be tough. I would have it hinged at the top instead of the bottom, then it's own weight would help close it. Other thing would be not to have the pressure too high as it would dissapate the fog too quickly to be seen. The light always on would work nicely tho..