A new
Weller SP23L soldering iron, an old soup can with the lid only partially removed or a heavy stoneware coffee mug (a shaving mug works well), some metal duct tape and a bag of smoke wood chips is all you need.
Clean the bbq and seal the vents with the metal duct tape. Punch a hole in the bottom of the can for the soldering iron barrel to go through, fill the can with chips jam the soldering iron in through the hole (make sure the "backstop" where the barrel meets the handle is not touching the can, otherwise you'll melt the handle on the iron), plug the iron in, put your food on and it'll be smoking within a few minutes. Every fifteen minutes or so, open the lid, shake the can to get fresh wood in contact with the iron barrel. Most foods will be done in 40-60 minutes.
If you use a shaving mug, just fill it with chips, stick the iron in, and stir it a bit about every 10-15 minutes.
In a 1 hour smoke, I get less than a 10F temp rise in the chamber of an 18" weber that I have dedicated for cold smoking.