I live in Alaska, when it gets cold it really gets cold, and stays that way for 7 months.To keep the cold air out of my workshop I needed to insulate the ducting and add a blast gate. Insulating was easy but I didn't want to hack up the ducting to install the gate.soooo.
What you'll need:
1.5" hole saw
duct tape ( the first time I've used it for it's intended purpose)
Location is everything, you want easy access. Drill hole, clean sharp edges, remove any metal shavings.
Line the edge of the hole with duct tape.
Tear off a 3" length of duct tape and fold over about 1/2", inflate a balloon inside the duct, twist the open end a few times and secure twisted end with tape. When the exhaust is in operation cover the hole with tape. I've been doing this for about a year now and can reuse a balloon 4 or 5 times before replacing it.