I've read extensively on the dangers of using anything galvanized where high temps are present and/or when in contact with the food.
1) I just purchased an Oklahoma Joe (triple) from Lowe's. I happen to have some thin galvanized sheet metal I'd like to use as a chimney extension. I rolled it up inside the chimney pipe then pop riveted it top/btm/middle. As the temps to which it will be exposed are not that great and will never be in contact with the food, do you see any problem with this setup?
2) The ID of the chimney is ~2.875". I can find 3" OD tubing in various materials, ducting, pipe, exhaust tubing etc.
3) Obviously, without modding the grate under the chimney, the extension will only go down to grate level. In theory, what is the optimal lower level for the extension? Would there be any benefit from cutting an opening in the grate to lower the level of the extension? I do not foresee the slight loss of grate area as an issue.
I have ordered some nomex gasket seal material to seal up the doors, will build a firebox > cooking area baffle/deflector and raise the charcoal grate level. I have a MIG welder and O/A setup as well and am not afraid to use either one.