Generally, the incieling speakers have adjustable tweeters, wherein you can adjust the angle of them towards the central part of the listening point. This itself should suffice, if one has followed the Dolby Atmos placement guidelines to an extent. My friend does not have any hard board false cieling and didn't want to install speakers with clamps / brackets. So he opted to use incieling speakers and made a wooden enclosure which had one side bigger than the other, so when the speakers are mounted they are at an angle and not pointing straight down to the floor. Please see Dali Alteco or SVS elevation speakers, you will get a better picture.
In your case, I think you want to incorporate a wooden box within an opening in the flase cieling. So if you don't have adjustable tweeters, you can have slightly different length of the wood, which will give you an angle towards the listening area. But if you have the option of having them in the optimum position, it wouldn't be required.
