the easy answer: draw a line and find a plexiglass place to do it for you.
alternatively, you can get sabre saw blades at home depot for plastic. tape the screen surface with masking tape to keep it from scratching while you work. try not to melt the plastic as you cut. this can cause the blade to grab the screen and start taking it up and down with the blade stroke and crack the plastic. clean the ragged edges with a scraping tool and sand to finish.
i've never tried a dremel for plexiglass but mine made very short, clean work of fibeglass.
I used a Dremel cut-off wheel (Fiberglass Type) to cut and mold my screen. The screen melted slightly but the melted area can easily be broken off once it cools. I cleaned up the cut (carved) area with the drum sander attachment for the Dremel. Be advised once you start shaping your shield, you'll find your self taking more and more off......