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Make your own pressure switch. Mount a normally open push button switch behind the grille. Make a flap out of aluminum flashing and mount it on a light hinge (like a piece of duct tape)at the top so that it will press the switch when air pressure against the aluminum flap is great enough. Wire your aux fan through the switch. Adjust the speed that it activates the switch by trimming the aluminum to the right size.
This is such a simple, elegant solution that you should figure a way to mount it on top of the hood so everyone can see it.
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