Looks like you have some good ideas to work with to get started......
I have a UDS that does not maintain temps very well either, and I know I have a few problems with it. Something you may consider looking at in the future, so I'll lay out my problems as I see them. I used 1" pipe nipples in three locations around the bottom. Each has a ball valve to control air. The ball valve chokes off the pipe pretty good, so is actually a good bit smaller than the 1" pipe (which actually is smaller ID). I built another UDS for a buddy and upsized the pipe and ball valves, and his runs real well.
For my charcoal basket bottom, I took two Weber Charcoal grates and welded them at 90 degrees to each other. I think this creates too small of holes to let ash fall through and contributes to choking off the fire. It is also a pain to build a circular fire basket that looks good. For one I built this winter, I used one Weber charcoal grate, then welded a piece of expanded metal on it. I then constructed a square basket out of the expanded metal 12x12x12 tall. This is more than enough charcoal for a long cook.
I have not looked yet, but my UDS uses a Weber kettle lid, and I think the exhaust may be a bit small......I like the Gateway drums and there's a reason they are popular. They run good, maintain temps, and can get really hot if you need them too......
Some things to think about......
All the best,
Ed