I build in place, no boxes at all.
I start with 1X4 framing in a floor support the width of the cabinet minus
the kickspace, joists front to back and a 1X4 forming the back of the
Then I add a ply floor the depth of the cabinet minus the face.
Then a 2X4 or 1X4 along the wall to support the top.
If I need dividers or drawer support, they are next.
Then the face frame, usually glued and pocket screwed together.
Then a few 1X from front to back to stabilze the face frame and add support
for the top.
Then the top, pocket screwed to the wall support and face frame.
The bottom edge of the face frame hangs 3/4" below the floor.
A 3/4X3/4 cleat is screwed to the back of the face frame and the floor ply
is screwed to it from above...could be glued and nailed.
This all becomed a strong and simple structure, with minimal work and