Carlon 1-Gang 20 cu. in. Blue PVC Old Work Switch Outlet Wall electrical box – $1.66 from Amazon with $1 no-rush shipping credit

https://www.amazon.com/gp/product/B00002N5FQ/
This is the same price Home depot and Lowe’s sell it for, but gives you $1 slow shipping credit.
This one is old construction, uses plastic tabs to secure in existing drywall.
Size for B120R:
4 1/8 Lx2 5/16 Wx3 5/8 D
Lowes link:
https://www.lowes.com/pd/CARLON-1…x/50046395
HD link: https://www.homedepot.com/p/Carlo…/202077323