CUTTING A HOLE IN TT DOOR? {{{{{UPDATES}}}}}}

I'd really be concerned about the structural integrity of the door if you were to cut a hole in it. Doors with windows generally have a frame under the skin with a reinforced "hole" the window sets in and the window's perimeter screws to. You may find nothing but aluminum skin and foam to fasten the window frame to. That said, you could probably insert wood blocking (I'd use oak, old pallets are good sources of strong but ugly oak) between the inner and outer skin. Dig out any foam in the way and apply contact cement (NOT the water based junk) liberally and slide the wood in while the glue is still wet. Make sure the recess for the wood is big enough and don't follow the usual procedure of applying the cement to both surfaces, letting it set up, then trying to slide it in. It will just stop at first touch. This should let you securely mount the window in the door without compromising the strength too much.