Patrick, I cannot tell you exactly which plywood they use, but I can tell you that it just appears as extra thick, very fine grade multiply wood.
Doubt that really helps much, and it also has the specs for steel in there as well, and certainly there are many more steel flam lockers out there then there are the wooden ones.
That said, some of the corrosive lockers that we buy are wood, as in some of those instances, the wood tends to hold up better and have less potential other hazards associated with it.
As another side note, flam lockers also are often vented, and I do not believe I have ever seen one that was "airtight" Most flam lockers I am familiar with have vent plugs (usually a weld in 2" bung) with at least one high, and one low to allow cross ventilation of the flam locker.
I would have to go back into the regs to find the citation, but I believe that one part of the thinking on the flam locker was also to keep the fire away from the flammables as in the temp inside a bit cooler and flames away, so that they would not ignite. Kind of like a two hour fire door.
Not sure if that makes sense or not and the way I am explaining it, but if you will, it gives the fire department a bit of time before the "fuel" gets added to the fire.
That said, the most straight forward thing I think folks should do is to identify where their problems are, and then post an NFPA 704 (that square on point red, white, blue and yellow thing you see on warehouses and such) with appropriate numbers on a hazard area. While it does not tell them exactly what they are facing, it gives them a heads up of the danger level that they should expect.
