wow, "the worlds first" open air case looks very similar in design to something i made in 2004, well maybe not that similar but the basic shape is there.
kinda glad i retired from the scene
For cutting windows i prefer using a jigsaw with a metal cutting blade, its cleaner and much faster than a dremel. just remember to use masking tape and clearly mark your cut.
you can find a 70mm round fan at allied electronics or a similar vendor.