I did this awhile back, when pre-modded cases were a novelty. It was my first mod, and at the time didn't have a digital camera.
Taking an abandoned REALLY old microwave door and cut about 1/2" of metal around the window.
I cut a smaller hole in the side of my case, drilled holes and bolted the two together.
A little left over body filler was used to smooth it out.
Cost: 1 cutting disk in my angle-grinder.