Can anyone recommend a website or place that sales a cast iron door with air intake built into it?
FIRE PIT DOOR
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You don't necessarily have to have a barrel.
They have several different styles of doors for the barrel kits on ebay, you could also check your local farm supply store, most of them sell barrel kits locally.
The barrel kit door seems to work well, and no sense re-inventing the wheel here, they have vents and are usually made from cast iron.
No they aren't airtight, but if you don't have some air coming into your firebox your fire will go out. Most have sliding vents of one type or another on the front to allow even more air in, for a bigger fire, or to control your fire. And even though its not airtight, when we close the vents on our barrel stove, the fire goes out fairly quickly.
Wow that ones is pricey....
Here is a link to one very similar to the one we have on our stove, if the door isn't airtight enough for you, you can always get a high temp rope gasket to go around the door.
It has the two vents on the bottom, AND the whole vent door opens for even more air, or to remove ash without opening the whole big door.