I used what is commonly called a blast gate. They are available in plastic or aluminum. They can be controlled (open/close) with a simple push-pill cable or a stronger solenoid. I used the cable to keep it simple, and the aluminum for longevity. They are three times as expensive though.

https://www.leevalley.com/en-ca/shop...CABEgKK4_D_BwE

Rather disappointed with the marine blower's air flow, so I installed a squirrel cage fan. It has four speeds so I control the flow rate.