I built a stand alone concrete block "box" with a used vault door on it.
Rebar every 6" drilled into the slab, then filled with concrete. Door
is a Diebold class A security 2 hr fire rated door from a records vault.
The local security company upgraded the door, sold me the used one for
$200.
Inside dimensions 6ftx7ftx8ft. Should have made it bigger.