i have also made my own external smoke generator. did the venturi style with the adjustable pressure aquarium pump. worked well, but didnt get the hours of smoke i wanted. still had to babysit it. the creosote build up was awful in the feed pipe and needed to be burned off every other smoke, or EVERY smoke to make sure things were open. the Smoke Daddy actually talks about the same problem. i also used square tubing, and found that it would also gum up inside and the chips, sawdust etc would not keep falling down all the time. needed to be pushed down now and then, put a spring in the middle like the smoke daddy but didnt solve the problem have since gone to an AMZNPS and have done a mailbox mod with a cheap turkey roaster. WORKS AWESOME !!!!!
i also have a MASTERBUILT cold smoke generator for $60 from Bass Pro that i have connected with 3" heat duct when i want to burn chips , or sawdust (my sawdust is too coarse for the AMZN) i also use it on my MES
just keep playing around and you will figure out what you want.
as for pipe..... go to a muffler shop and get some 4" exhaust pipe from a big diesel. its not galvinized, its a mix of aluminum and steel !!!!
there are some pretty good pics of my smoke gen on this thread of my build.
GOOD LUCK !!!