BTW, maybe you meant this by 'not square', but really it means non-parallel walls. If it has to be parallel walls, then obviously having no two axes be the same length or multiples of any other shorter axis length is good. But really you want non-parallel walls if you can. If you can create a non-flat ceiling that's good, and usually gets rid of a hard wall/ceiling corner. If you can make the two side walls not be parallel, that would be really good.

If you get rid of the four corners that's good. You can actually get double duty there. Round them inwards (like a section of a small column stuck through the wall), and stuff that shape with rockwool to absorb bass reflections out of the corners as well. The rounded shape will diffuse the more mid-range frequencies, but the low end stuff will just go straight through it unless it's made of solid concrete or something, which it won't be in your case. So it's a double duty way to get absorption and diffusion in the corners, and it can look nice as well according to how they are done.