I have a bunch of in-wall speakers going in the basement ceiling. The spans the speakers are going in are between engineered I-beams with a 13' length. I would like to build boxes to enclose the speakers in the ceiling as two speakers in the open 13' span won't sound good.
Am recommenations about the optimal size for the enclosures and any padding/sound material in them to stop resonance?

Thanks in advance :)