You're on the right track in going to a metal box...it will slow things down a little on smoke output if you are experiencing heavy smoke for long periods. You will need to experiment a little with how much you add to determine how long it will last. Also, placement of the box in relation to the heat source does matter. To regulate the smoke, you need to find that sweet spot where you have enough heat to get smoke, but not so much that the smoke wood burns up too fast. Air getting to the smoke wood can be regulated with covers (similar to using a foil pouch), but if you can regulate the heat works best for getting longer smoke times with less wood.
Stainless is a great choice if you can get it...cast iron will take longer to heat up and produce smoke, and also dissipates heats slower than other metals. This may be a better choice for a non-thermostat controlled heat element, or a heat element that makes large temp swings (longer on/off cycles) to maintain your average chamber temperature. Cast iron will hold it's heat longer during off cycles for more even smoke output.
More heat and/or air = more smoke, less heat and/or air = less smoke.
If foil pouches are used to slow the burn of your smoke wood, it can't be in direct contact with the heat source. Lava rocks over the heat element will help with that, and placement of the smoke wood a bit off to the side in stead of directly above the heat source also helps.
Anything else comes up, just ;let us know. Good luck and good smoking!