Will it be solidly mounted, or will you suspend it somehow? If I can correctly picture what you mean, I think I would try to hang it, instead of solidly mounting it, to reduce/eliminate the possiblity of vibration in the ceiling. I would just leave a very small gap between the edge of the baffle and the ceiling opening, then install some trim around the opening.
Well! Just
listen to me!
I haven't even finished my
first DIY subwoofer yet and I'm telling you how to do your job! Just ignore me...