I had one window fan already, but I wanted to add another 80mm to help cool my video card.
I used an 1/8" carbide cutter attached to my dremel to cut the hole. I put like 3 layers of masking tape crossing eachother on both sides. It didn't leave one crack or anything in the plexiglass, so that's good.
I know it's not very round, lol, but I did the best I could.
It may not be very pretty, but it dropped my videocard board temp about 10*C, and dropped the GPU like 6 - 8*C at idle, I haven't tested full load yet.
What do you think?