Harry, if you want to use the steel lockers you mentioned, they should be stainless or bare steel. Some form of enameling etc. is suitable (as used in commercial barbecues) but regular paint and probably galvanizing also as said is a no go.
Finnish way of smoking means you have somewhat airtight container where you put fish/meat on wire trays and in the bottom the chippings of the wood that generates the smoke when you heat the container from below. The chippings wouldn't be actually burning but smouldering (sp?) because there's no oxygen (air) being fed to the container. You control the heat somewhat with the fire underneath the container, but the process is always quite quick (like 15 minutes from small fish).
It would be possible to lower the heat by separating the chamber where chippings are put and connecting it with a pipe to another container where fish/meat is put. I have only seen this when cold smoking, and then the pipe could be like 10-15 meters long and buried in the soil. In cold smoking you absolutely cannot go over 40 degrees Celsius (proteins start to coagulate so it's not cold smoking anymore, although result can be very good). This process would last something like 10+ hours but the fish/meat is not in very thick slices, maybe an inch at most.
There's also a process called "palvaus" or "palvaaminen" which is a traditional way to cure big chunks of meat. The temperature is around 100 degrees Celsius as it's done using savusauna (i.e. smoke sauna). You can introduce smoke or not, of course depending on your heating procedure there's always a little smoke since savusauna doesn't have a chimney but you can add the chippings etc. just like when smoking. I guess the wood should be fresh so it would generate a lot of smoke because otherwise it would just burn away as there's plenty of oxygen available. Palvaaminen is like cold smoking it takes 10+ hours but you can use hams etc. so the meat wouldn't have to be cut in slices.
Nowadays there's electric devices available which make especially the cold smoking much easier. Since they don't introduce huge amounts of heat there's no need for long cooling pipes. In fact you can cold smoke using a carton box if the outside temperature is suitable (say 10 degrees Celsius).
Enough of Finland, if you want to use hot ambers in the same container as the meat, that would be barbecuing with indirect heat. Just use lump charcoal and make sure the meat doesn't get any direct heat. You can put the charcoal on one side of the barbecue and meat on the other side. Put a metal tray below the meat to catch any drippings (little bit of water in the tray is a good idea). Then yo ucan add chippings on top of the charcoal from time to time. Depending on moisture they burn or smoulder but add the smoke flavour anyway. I have also seen a version where metal tray was put on top of the charcoals, chippings in the tray and meat above the tray. The chippings would smoulder not burn because the heat was lower (and the tray itself shielded the meat from direct heat). Indirect barbecue would take something like 2-5 hours depending on meat and your preference.
If you cold smoke it is less likely to become unpalatable!
