Hi Matt:
We have a metal back box, the BK-BB6. It is for new construction where the walls are open. The Box opening is 7.75". The cutout dimension of the ACE640 speaker is 7.80". The boxes sell for $49.99 each.
All of our In-Ceiling and In-Wall speakers feature infinite baffles so they can work without a back box.
Here is a link for a guide to DIY building a Soundproofing box for speakers: https://soundproofguide.com/diy-soundproofing-in-wall-in-ceiling-speakers/
TimL/12-8-2022