I think it would also help if you turn on manual exposure and leave it at a fixed value as you take the pictures. Brightness between images shouldn't be as much of an issue then.
Done!
This isn't a 180 or 360 degree field of view, but it's fairly wide.
(Linked to a 1680px-wide unframed version.)
The stitching is a bit bad. (One is more obvious than the other.) I need to get a heaver tripod.