Looks to be 18 gauge sheet metal. 54" wide, 15 1/4" tall, 6" deep at the bottom, tapering to about 2" at the top. There is a 2" shelf lip all along the bottom, with a break for the angle starting about 1 3/8" from the bottom.
It could probably be made out of wood if you braced the corners. A lot cheaper to buy from the junkyard.
Gary,
For the record, I will have two gallons of two stroke oil (my preferred lubricity additive until I can find a local supplier of biodiesel), a few tools, a gallon or so of water, a funnel and some rags. That will still leave plenty of room for parts or ammo storage.
There are a few holes punched out near the top, looks like a body could store a couple of rifles back there, too.