Convert Outline of a shape to a curve

My objective is to create a curve (let's keep it simple and say a rectangle) 8 pts thick. In the past I've created a black fill rectangle then a smaller rectangle inside 8 pts smaller with a white fill, followed by a back minus front.

Seems to me there has to be an easier way to do this to create that 8 pt shape. I've created a rectangle with no fill and an 8pt thickness for the outline. Is there a way to convert this to the same kind of curve as above? Simply converting to curve doesn't do it.

I need this for vinyl cutting. Wouldn't matter for a positive for screen printing. But for vinyl cutting I need the curve inside and outside the shape for the plotter to cut this correctly.

I'd appreciate your wise counsel.