I managed to get the plenum cut out and tacked together yesterday. The smokestack is a length of 4.5 inch pipe, or 114mm as we Brits like to call it in new money. A couple of years ago i was helping clear a garage out and I found a bunch of 1/4 inch plates, I kept them because I was sure they’d come in handy - I’m glad I did, so far that plenum is the cheapest part of my build.
Joking about the metric/imperial thing, I work as a LVC in the U.K. for an American client. I can quite easily jump between feet/yards and metres, but when it comes to fractions of inches, my brain simply cannot compute. The smoker build section of this forum is the only forum I have to read with a calculator beside me !