I have very little experience working with plexiglass, but this is what I did:
On the square cuts, I let home-depo score and snap the plexiglass to approximately the correct size. As could be predicted, it was a good 1/4th inch too big for my cab. I had already destroyed a piece of plexiglass by scoring it badly, so I decided to try something creative...
I mounted the plexi between 2 pieces of scrap wood with the plexiglass sticking out just the amount that was undercut. I clamped the 2 boards and glass together very tightly. Then, I used my belt sander to sand the plexiglass to the exact shape. This allowed me to make *very* precise cuts with absolutely no hairline cracks.
I wouldn't try this on more than 1/4th an inch at a time, and even then, work at it slowly.