49 CFR § 178.514 - Standards for plywood boxes.
(a) The identification code for a plywood box is 4D.
(b) Construction requirements for plywood boxes are as follows:
(1) Plywood used must be at least 3 ply. It shall be made from well-seasoned rotary cut, sliced or sawn veneer, commercially dry and free from defects that would materially lessen the strength of the box. The strength of the material used and the method of construction must be appropriate to the capacity and intended use of the box. All adjacent plies must be glued with water-resistant adhesive. Other suitable materials may be used together with plywood in the construction of boxes. Boxes must be nailed or secured to corner posts or ends or assembled with other equally suitable devices.
(2) Maximum net mass: 400 kg (882 pounds).