how to create a smaller inside outline of a shape

I am trying to create an "inside border" to a complex shape.  Because it has many different tabs and recesses, I cannot simple copy the shape and shrink it.

If I make a copy, and then increase the stroke to 15 pt, the inside of that shape would work. But I don't know how to create an "inside outline" of that shape, or tranform it into a shape by itself.  Can anyone help me?