These speakers are designed for walls. Our walls and ceilings typically have more than 3ft³ of airspace in them. Most in-wall and in-ceiling speakers are Infinite Baffle design for this purpose.
When you constrict an infinite baffle design’s airspace behind, you basically squeeze the speaker from behind and limit the movement of the woofer (the larger speaker on the plate) and that lowers the amount of bass (low sounds) coming from the woofer. That’s why you shouldn’t build a box for these speakers.
|Long answer, see this article: