Just in case this helps. Note I'm at high altitude but if I can get it to work up here I have faith you can as well. I've been having the same issues as you stated and I was at my breaking point with it. I had made the mailbox mod with no reliable luck either. I could get it to work for a couple hours but always went out. Decided to give it once last shot with two more ideas:
Idea #1- I thought it could still be airflow so I was going to use a fish tank pump to add oxygen to the mailbox(already using a extension cord for the smoker so power is no issue)
Idea #2- one more try at placing it in the smoker. This time I did all the same steps (microwave, torch, blow out, blow to get fire, blow out). I also completely removed the chip tray, and took the drip pan(located in the bottom left) out of the slots and placed it on the floor under the opening. I placed the now smoking
AMNPS on the lowest rack and directly over the drip pan opening in hopes to increase airflow. I make a little TP over the pellets so they don't get put out by drippings.
Results- never had to try option #1 because option #2 has worked like a champ, I've had three successful 10+ hour smokes since trying this. I was worried about the chip tray being completely out and losing heat, but it's been average of 20 degrees my last three smokes and no issues with temp. Its currently in use so I only snapped a quick picture of what I'm talking about but I'll look and see if I have anything better.
Hope this helps, it's easy to get so dang frustrated but there's always something new to try, and the smoke it produces is hard to beat in a MES!
Kyle