A diffuser is used to scatter sound waves unevenly so the surface needs to be also very uneven. So you need a mildly reflective material such as wood or cork cut out so that it has edges and crevices of different sizes, angles and shapes. This will cause the round or flat (depends if we're talking near of far field and consequently the size and shape of the room and speaker placement) to break into frequency components that are reflected to different directions. As a result, you won't be able to hear individual reflections as easily.
It also breaks the symmetries of the room to a certain degree, especially if it's curved and placed between the ceiling and the wall behind the listening position, and thus stops some standing waves.
Unfortunately, I can't give you an answer in terms of formulas or certain designs for two reasons. a) I don't have enough information (like the purpose of this room, listening position, internal shape and the materials inside it) and b) I'm quite rusty with acoustics and I wouldn't want to tell you something that isn't correct. It's also the kind of thing that is done with specialized software rather than trying to make approximations using pen and paper.
There are companies that specialize in acoustics and they should have the software to suggest a certain shape/construction.