I actually cut them with my 6.5" cordless circular saw. I laid the plexiglas flat on a scrap of MDF and set the blade depth about 1/8" more than the plexiglass. The cuts were pretty decent.
That gave me the rough shape for the windows. For the curves at the corners I marked the curve with a sharpie on some masking tape and sanded them to shape with a 100grit sanding disk on my 4.5" grinder. It worked really well. Then I used the same sanding disk to smooth out the rest of the edges. I finished off the edges with 220grit on my RO sander.
I was worried about working with plexi, all of my previous interactions with it did not go this well. Now I just need to remember to be nice to it so I dont scratch it. Lol.