First I measure 1 & 3/4 inches all around, drew lines & then taped all the lines.
I then used a dremel with a cutting bit No.561.
After the cut I used a mini electric sander with 150 grid, to sand all the lines smooth.
You can buy small like sanding cylinders 1/4 to 1/2 inch, thats what I used for the corners.

Plex glass you just cut 1 inch on each side bigger then the hole you cut, then apply your 3M double sided tape.

If your cutting for a fan all I did was use a 80mm for a template, marked it ect.
How to get the hole round is easy, just use something like a cone & glue sand paper onto it, as you place the cone into the rough out cut hole leaving just a small amount away from the hole. It will take no time & thats how you make a perfect round circle...