Yes it puzzled me how I was going to cut the windows out as these tinny PSU cases, (monkey metal), bend if you look at them the wrong way
What I did was mark out accurately, (using a scribe), the window dimensions. I drilled a hole in each corner with the appropriate drill, (think it was 6 mm), so the outer arc of the hole just cuts the corners of the rectangle I'd scribed. This gives the neat corner shape. I knew that the back a fourth motion of sawing was not going to work so thought a mini cut off wheel in my Dremal could do it. This is what I ended up using, (was actually mini drill cut-off wheel in a Dremal)". With care a patience you get a pretty straight cut, have to be carefull not to let the wheel "grab" the metal or it will bend it. When the window was cut out I clamped each side in a vice supporting with a few strips of alu so that I could file the cuts neatly without risk of bending it, a bit fiddly but turned out ok in the end, and like most things pretty easy when you've done it.