mesh is technically a one-sided plane. It has texture on one side (also it's "solid" on one side). The other side is drive/shootthrough, and clear. In other words, there is no way to tell the mesh is there if you look at it from the "back".
i'm not sure about how to turn a mesh face around in Wings or blender, but in Sketch-up there is a simple-to-get-to command "reverse faces".
so in this case, the outside box will let the bullets out, but keep other bullets from coming in, while the inside box will keep the tank from diving out through the side. Drivethrough top of the box lets tanks in/out while keeping the shots (like GM, or if "_shotsKeepVerticalVelosity 1").
talks more about the Mesh, and gives a couple examples that can be useful in the situation here.