Deftech makes some angled in ceilings that would work. They are rectangular so you can't aim or toe them in. So, that means place them withing the width of the seating. They have a couple models. These are the least expensive.
[Link: definitivetechnology.com]

I think Deftech has good options for surrounds as well as in walls for the LCR and backs if needed, just make sure you mount them vertically and not like a center channel like they show on their website.