Baffles are RAD!
I experimented with a couple cookie sheets until I found one that would fit nice over my firebox hole, and stuffed aluminum foil in any gaps. It's not a permanent mod, but it's really easy to clean and will be a good stop gap while I figure out something a bit more erudite.
I agree about the firebox hole being a little big. My chargriller's portal is about half as big and still needed regulation to avoid 45-60 degree temp differences, even with modding the smoke stack down to about the same level as yours.
I'd unbolt the top three bolts holding your firebox on, (not all), measure out a piece of non galvanized sheet steel (Not aluminum
!*)) to forluvofsmoke's dimensions and so the bolt holes line up, and bolt that on inside your smoking chamber through the existing holes.
About seven minutes of work with most basic power tools.
*Aluminum burns. Any metal burning bonds with oxides and puts of some nasty nasty substances. It might make a great stove top pot or pan, or outer shell for a smaller smoker, but there's a reason they don't make decent woks, or kilns out of aluminum.I work sometimes with some of less sophisticated race vehicles, and I've seen aluminum alloys catch fire.
I was warned against using a galvanized steel garbage can for a UDS, and for good reason. Zinc puts off some nasty oxides when heated to the temps of burning charcoal. But from my experience aluminum isn't the best for exposing to flames for an extended period of time.
Besides steel is WAY cheaper. Just my 1.634 Cents (after tax