Finally! After many hours of working on this beast, the water heater cabinet is finished at last! I've finished attaching all drywall panels and have applied drywall mud and drywall tape where the panels meet.
As a novice in working with metal, I'm reasonably pleased with the outcome of this project. It was difficult and required some re-engineering from the original design, but it turned out very solid.
I was originally going to skip painting this cabinet, but I've learned that the drywall will darken over time and I'd rather that the unit blend in well with the rest of the garage interior. I'll be painting this a flat eggshell white in a few days.
So, with little delay, I painted the cabinet a basic flat white. Not bad for an evening's work.