If you're just cutting drywall in the ceiling you should be able to fit some MZerotis up there. They're only 4.5 inches deep. Most ceiling beams are 6 inches I think. There should be plenty of room between beams also. You could make mounting brackets that snap into the grill holes (since you'll remove the grill anyway) and simply have the speaker sit on the top of the drywall.

Then use a generic white grill attached to the drywall.

Frank