I just completed mine.
Make sure the mailbox is lower than the piping going into the smoker. Open your vent all the way open.Once a vacuum starts and the smoke gets rolling it will continue on. I dont microwave my pellets anymore. I put them on a cookie sheet and in the oven on 170 for about 1/2 hr or so while the smoker is warming up. They light well and burn the full time this way,
I added a row of 3/8" holes on each side of my mailbox. I also put the exit on the rear part as high up as I could get.
When I first start the process the smoke seeps out the holes and cracks in the ducks. But once that vacuum takes hold she goes real good and smokey. No more stuff dripping putting the smoke out. No more worrying about the moisture in the smoker slowing the pellet burn or putting it out. Indirect heat for smoking cheese.