Could you build cabinets around in ceiling speakers?

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.

Understanding Speaker Data
Wikipedia

-Robert

The joy of software. If you look in this forum you will see posts from users of several different programs, some free, that allow you to model different enclosures. WinISD is one -http://www.linearteam.dk/default.aspx?pageid=winisd

While you can measure the TS numbers yourself the more common way is to use figures published by the seller or available on a forum. You can dump these into any number of software programs that will spit out box size options for sealed systems (the easiest to build or ported systems and other types that are a bit more difficult).

There are also some books and many on-line references as well as folks here and in other forums that will be happy to work with you, if you get into DIY.

And, they won't forget that Thiele-Small is not exactly a household term . Using software avoids having to wrestle with stuff like this--