And where abouts should I cut the door? Looking from the end of tank, at like the 9 and 12 o'clock positions?
Ok, this is a hard one to advise someone else on..
All I can say is:
The smaller the door opening, the more structurally stable it will be.
The smaller the door opening, the less heat you will let out when you open the door.
The smaller the door opening, the more pain in the but it will be to load and unload and clean out the cooker.
So, draw it out , look at it from the finished height that the cooker will be at. Think about the kinds of food you will be cooking and loading up on the cooker.
Personally, I go just slightly (1/4"-1/2") higher than 9 position so that less juice runs down the outside of the cooker. Then I go high enough so that I can easily see the rear of all of the cooking grates and high enough up the back wall as would my tallest meat would be.
So, on a very large cooker, it could only be from the 9 to 11 position, on a very small one it could be from 9 all the way to 2.