The problem with opening the door is that the opening is the full height of the smoke box - and the smoke will therefore take the easiest route out. You need to create the environment where the heat produced by the smoke generator rises and flows through the pipe causing a natural air flow up and out through the top of the smoker. In order to do this you need to restrict the air intake in the firebox to the bottom quarter of the door.
Thanks for the reply
The door has an adjustable catch so can be left open to feed air, but when I do that the smoke just falls out of that opening.
The only way to create draw it to offset the roof of the smoking chamber which doesn't work that well. I may try adding a chimney at some stage.
It worked fine with the fire so I guess the problem is with cold smoking. I might invest in a cheap battery fan to put in either chamber ( even the smoking chamber gets barely more that worm)
A work in progress
Before you make any holes in the door you may want to do some trials. Make a temporary door out of card or plywood and tape it on in place of the door. You can then cut a small air vent towards the bottom and experiment with the size before making any holes in the door..