When laser-cutting, the laser beam has a width (referred to and the “kerf”), so you lose a little material either side of the actual cut line.
The laser cutting place will tell you the approximate kerf size of their laser cutter, so you can compensate by adding half the kerf width to the dimensions of your window.
However, if your panel is pre-cut, you don’t know the tolerances of the manufacturing process that produced it, so you won’t know the exact size of the hole unless you have a micrometer to measure it yourself.
I’d recommend getting a few windows cut at slightly different sizes.
You may still need to sand down the nearest fit to get it to press in securely.
