Draft block/damper on hood exhaust ducting?

Since I live in north eastern Washington state, I am trying to figure out how to block the exhaust duct to keep cold air out when the hood isn't in use.

I would like to know what others in a northern climate have done for this potential energy dump.

Thanks,
Dave

I live in MN and am used to the attempts to keep heat inside. I would think something as simple as an in line dryer vent flap would work.

Agreed. At lowes or any hardware store you can get either a damper where you manually close the line or a 1 way exhaust where air can only go through one way

I am using a 6 in duct (actually a sidewall thimble in place from a wood stove chimney), so a dryer flap won't work.
All the sealing vent caps I have found are really flimsy, light weight, not insulated and don't seal reliably.
I'm resigned to building something that I have to manually open and close. Just hoping for some good ideas or learn from what hasn't worked from others on the forum.

A few years ago a neighbor installed an 8" exhaust fan through the wall in his kitchen. The vent had a spring loaded flap that was pulled closed by a chain and opened when the chain was relaxed. It turned on the fan automatically when in the open position. I think that it came from a commercial kitchen sales place.

I bet something like this would work.
http://www.pexsupply.com/Fantech-RS...draft-Damper?gclid=CNapr67JsLQCFcKPPAodAFYAxw