90 angles aren't the problem. People treat corners because they are the highest pressure zones, and thus trapping in the corners becomes the most effect--it deals with 2 and often 3 of your axial room modes. We often use soffits with single layer sheetrock. 90 degree sections are still there, but you have disrupted the area of the highest pressure and distributed the pressure and thus reduced the peaks and lowered the Q factor for the peaks. That's probably what you meant--but I just wanted to clarify it a little further.
As to the ceiling, RPG skylines and hemifusors are both excellent products and easy to mount. The downside is that depending on how much surface area you are covering it can get expensive.
As to the ceiling, RPG skylines and hemifusors are both excellent products and easy to mount. The downside is that depending on how much surface area you are covering it can get expensive.