A lot of times, in wall speakers are a free air design, meaning they don't like enclosures. You'd be better off starting with a DIY kit of drivers and build the crossover and encloruse yourself.
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I would check out audioasylum.com "high efficiency speaker" forum and diyaudio.com "loudspeaker" section. Browse, post a question, etc.
Also, Madisound.com for speaker kits and Fostex single driver speakers (like Omega speakers). They have plans for many of the Fostex drivers.
Warning diy speakers can be very addicting.
I recently heard a pair of Brines Acoustics FT1600 Mk II's. These were manufactured speakers that retailed for $900 plus shipping. They are also available as 'flats' (precut cablinet sections ready for gluing) and pre-assembled cabinets or just the plans for far less. They are a single driver design and sound as good as anything I have ever heard. I just ordered the flats. They are also very efficient. Too good not to look into.