Old Work Box

For cutouts in plywood, as opposed to frame mounted receptacles.

https://www.amazon.com/Lamson-B120R-Carlon-PVC-Outlet/dp/B00002N5FQ

Allowed in New Construction

https://forums.mikeholt.com/threads/installation-of-old-work-boxes-in-new-construction.61773/

Exceptions

From [1]

Just a heads up, if the wall is fire rated the box must be secured to the stud regardless of the box listing. This is a building code issue not a electrical code issue.

I have no idea if the opening posters wall is fire rated.