I built these as a place to stash my gear out of sight from bad guys, prior to my safe. Box constructed from 3/4" shop grade plywood, interior dimensions 12.5" wide x 7" deep x 43.5" long. Overall length 56". It may be hard to tell from the photos, but these box's mount betwen floor joists (16" centers) and are covered by a piece of 18 ga galv sheet metal as camoflage. If you install them in an unfinished basement (for example) and butt them up to existing SM duct work, they blend right in. They mount on a hinge point at one end, with a spring loaded latching bolt at the other. To deploy, just pull a hidden cord attached to the spring loaded bolt and hinge down. I have a considerable ammount of time and materials invested in these, but now they are taking up space in my garage. Free, made from 3/4" plywood. inside outside installed (prior to false SM skin), shows spring loaded bolt mechanism engaging existing ductwork.