Not wanting to step on the other thread about cold smoke smokehouses, I generated this one.

About 2 yrs ago I made this attachment for my BBQ grill, from scrap items I had laying around my shop/garage. I did purchase the fish tank air pump and soldering iron (total cost @ $20 total)

Here are the pics:

Basically, I place the soldering iron in the hole in the side of the attachment, open the top of the attachment, fill with smoke pellets/chips and plug in the iron and the air pump. As the iron heats up and generates smoke the air pump pushed it into the bbq grill and fills the entire thing up with smoke.
Cheap, simple and effective

“Two things are infinite: the universe and human stupidity; and I'm not sure about the universe.”
― Albert Einstein