Ceiling would be ok, but I'm trying to keep the work down. And wall speakers would let me mount them at a good height.

I found these, which have a cutout dimension of 5", but frustratingly I can't find them for sale anywhere.

Those would be the absolute maximum width, too. I measured again, and the width of the area I have to deal with is just barely under 6". But that goes right up to the wall. So the actual width is probably a maximum of 5" for the cutout, as long as the finished width isn't too much greater than that.

*edit*
Ugh, I called the only site I could find that still listed that speaker and he said it looks like it's been discontinued. Fantastic.