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None, or minimal. The holes are about as big as pencil.
I'll take a couple pictures and post them soon as it quits raining. It worked like a charm for me.
I also painted the edges of the new holes with automotive header paint (header or engine paint is WAY cheaper than any other high temp paint I could find) to prevent rusting.
I'm not sure it would have but better safe than sorry.
Is there a diffuser plate above the flame and smoke inlet to the smoker body.. The rapid expansion of air, from higher temps, could be reversing the air flow... also, the air currents from the burner are exceeding the exhaust...
Dave wins the prize!
I had the exact same problem as described by the OP with my old Smoke Hollow propane smoker.
This one has no drawer - only the single main door, and only one small vent on the back (centered near the top).
I had added a 3" elbow, entering the RH side of the smoker just above the chip tray, for connection to the mailbox duct.
Smoke would NOT be drawn into the smoker from the mailbox duct as long as the burner was lit - period.
I have to agree with Dave's comments as highlighted above - thus, more venting capacity was needed.
Today, I dragged the old gasser back out of winter storage and cut around the original vent on the back, sizing the hole to accept a 4" galvanized start collar. I used a hand-operated sheet metal nibbler as I didn't have the correct size hole saw. The start collar is easy to install once the proper size hole is cut. I suggest using a clamp to fully seat the bend tabs for the most secure attachment and minimal (if any) leakage.
I then attached a 4" aluminum elbow and a 24" all-aluminum dryer vent hood for a stackpipe. (I removed the vent flapper, of course).
A test run using the pellet tube in the mailbox produced good smoke from the vent hood immediately, and it continued to work well from the very lowest flame setting (needle valve mod) to the highest flame possible.
I am more than delighted with these results.
Hopefully others might benefit from this information as well.