Every piece is framed. Some of the decking pieces slot together, like the ones you see in the rear. The center rear piece is hinged to the piece in front of it and there are metal tie-downs on either side of the medical supply box that attach with locking hinge pins to points welded to the framework and to the bed floor.
The side pieces slide into channels of the center piece and are also attached to the rollcage via locking hinge pins. When the gate is closed, the slant of it also holds all three pieces down at the rear. The gate is locked on both sides inside the truck to the rear quarters.
The rest of the deck is a combination of the above, with added locks in the centers that lock to 2 removable roll cage braces that go across the bed.
The heavier stuff under the deck is secured with straps under the deck to points either on the rollcage or in the floor of the bed, to (hopefully) keep stuff from flying around in an accident. I still want to add some extra hold down straps of some sort over the entire deck, but I haven't figured out anything that I like yet that will look as seamless as the deck does now.