For John Schroeder:

John:
Thanks for the info.  Unfortunately, I need a 4 inch shutoff.  I did talk to the mfg of your 3-inch shutoff but it was too expensive for him to make a one of a kind (4-inch) prototype.  My A/C condenser needs a lot of cooling to handle high ambient temperatures on the ground.  Thats why I enlarged the plenum and went to the 4 inch duct and blower.  In my original setup, I used a laundry drier shutoff coupled with a 12 volt selenoid from a vintage Firebird but it was just too lightweight.  Appreciate your interest.  Regards, Bill H.