Fridge/Fermentation Air Exchange Questions

I am building a walk-in fridge and want to append a fermentation chamber to the side of it in order to use the cold air. So, I’ll need an air input and return (3 or 4 inches) and a blower controlled by a two stage Ranco Thermometer.

I am concerned about effectively sealing the air flow when the thermometer indicates the target temp has been achieved (or is in heat mode).

How can I easily seal both sides of the air exchange? I’ve read a few posts where people had issues with this.

Also, if you have experience in this, what sort of blower fan would you suggest?

Thanks

Whoa, there's no suggestion?