My brother in law is german, and got an old wood burning boiler from his father when he renovated his water system. It works just like a kelly kettle. Well he started cutting the boiler up, he chopped off the top, and the very bottom, and removed the chimney that runs up the centre. He then drilled holes at the top to put his hanging rods through. As they are threaded bars with nuts on the outside to hold them in place, he can remove them if he wants to smoke larger joints, etc.
He then got a large butterfly valve welded to the cut off top section of the boiler, which he balances on top as the lid. This is how he controls the smoke in the boiler, and it also ensures that the fire at the bottom of the boiler doesn't flare up, but smolders.
Due to it being a boiler, it has a small temperature guage on the side, and this helps him guage when to start certain stages of the smoking process. If the fire starts to burn, he smothers it with sawdust especially made for the job.
A very good use of an old bit of kit that would otherwise have been thrown away. I have had some of the trout he has smoked and it was excelllent. I get smoked trout from the shops here in germany, but it doesn't compare to fish that was caught that morning and smoked in the evening after a day being marinated in a secret recipe!
Anyway, a good thing at home, but not the sort of thing you'll take on the trail with you! How do you improvise in nature?