Most surrounds are run as small, when setting up the AVR. That means that much of the spectrum that is most effected by cabinet design is not going to go to the surrounds. Thus the impact of using a box is limited. There is not too much point in trying to improve the response below 80 or 100 Hz if your bass management settings are steering that range to your mains or sub.
Some in wall and in ceiling speaker makers tell you what volume of air they assumed in designing the speaker, and some models come with enclosures designed to fit between studs or joists. Others tell you nothing.
If your speakers were in a ceiling affixed to 2x8 or 2 x 10 joists on 16" centers you could easily fabricate a MDF or plywood box to fit. The question however would be how big and what design? If you knew the TS parameters you could get lots of help designing the box, but you would have to know the TS numbers.
There are many in wall and ceiling speakers that are clearly intended more for PA or background music, just as there are also good ones. Placing speakers in the overhead is a room layout or customer preference driven decision. I've heard some pretty good setups and some stinkers.
A better way to approach this might be to figure out how much you are willing to spend, then do some research and then come back with something concrete in terms of makes, models and TS figures. Then you could talk cases.
