Cut the plywood to the dimensions that you need. The interior of the steam box should be large enough so that when you load it up with wood there will still be an inch of space around the wood so that the steam can get to the wood.
Nail or screw the 4 sides together.
Close off one end of the box. Leave the other end open.
Screw a piece of plywood that is 4 inches wider than the diameter of your cooking pot to the capped off end of the steam box.
Cut a hole in the center of the piece of plywood so steam can get from the cooking pot into the steam box.