SUCCESS!! When I turned the thermostat down a little last night, the temp dropped to 375 as planned in about 45 minutes. I turned it back up to what I thought would be about 425 and went to bed. When I woke up my son was playing a drum and yelling I'm a bubble monkey, I'm a bubble monkey! Wait, that's not my point.
When I woke up, I went downstairs and the Chubby was at... 425! And, what's more there was still plenty of coal in there to get it going for the day if I wanted to, which I don't cause it's August! Of course I was only running at 425 overnight. 500-525 overnight might be a diferent story, and i still think a hopper would help this stove, but hey... so far it appears to hold where I set it without issue.
Next step, paint and finish the draft box, add maybe figure out how to better mount a gasket to it. I've got some RTV silicone I used to put a gasket on my pellet smoker door, but I still wish I'd built a gasket channel into this thing.
...And I'll post more pictures of the finished setup.