The roof hatches MCI used are a different dimension than school bus roof hatches. Instead of square they are more rectangular. You may want to contact Luke at US Coach or Sam at Caylors Supply to see about a used MCI roof hatch.
I have an Avion that has non-standard sized roof vents. The OEM replacements are made of unobtainium and priced accordingly. Instead of reducing the size of opening I used a piece of Lexan to cover the hole. It has been up there more than five years and looks exactly like it did when I put it up there.
The one real upside is it really lets a lot of light into the coach. The one real downside is it really lets a lot of light into the coach.