What is the best way to add another blow hole for a window that is already installed in the case.

First draw an outline of the blowhole. The scotch tape then outside of the outline. Then add constuction tape over that. If u have a big 80mm or 120 mm cutter just drill it. If your using like a rotary saw like Dremel leave space for the disc to turn. And with the excess just sandit off to a nice circle. Thats my look at cutting a blowhole.

i would say get a holesaw and a drill and be done with it but i have access to that kind of stuff for free where i work

go slow so u dont melt it.