if you want it exact them make a new layer and place it at the very bottom of your layer list (so it isnt visible yet) or you can click the eye to make it non-visible. Then fill it black, take your ploygon selection marquee tool and select the areas you want to keep, invert the selection then delete so you are only left with the border area instead of the entire black frame. then re-select that area, copy, paste and use free transform to flip it vertically, then move it down to the bottom (or top depending on which you did first) using this method ensures that the one you copied is the same size and dimensions of the first one, so basically a mirror of the original which looks alot nicer than having 2 that are different sizes :/