Most speakers are designed to be used in a freestanding setting, so you'd have to equalize them, likely a tilt down below 1 kHz or so. The dimensions would accommodate a smaller tower speaker. For example, the Revel F35 *almost* fits perfectly. The PSB Alpha T20 and Imagine X1T would, but you'll want to fill any open space around the speaker as much as possible.
Unfortunately, I believe that most in-wall speakers are unlikely to fit the length and width dimensions. Also, studio monitors, which can be amenable to flush mounting, also do not typically come in this shape.