I would keep the firebox where it's at and just mount a plate to restrict the opening to see how it does, but if you had plate fab resources, then your idea sounds like it's easy enough.
- My Custom Vertical Offset Smoker by AJ's Custom Cookers (build link
- Superior SS-2 - Courtesy of Gainesville Jaycees and Superior Smokers
- Weber Kettle 22.5 OTG
w/ Craycort Cast Iron Grates
- Mini WSM (from SJS)