Cutting plexiglass with a utility knife works best for making straight cuts in thin sheets of plexiglass (1/8 inch or less). But while a utility knife is convenient (you probably have one in your tool ...

The dremel tool worked well for me, but what ever powertool you use to cut it will also melt the plastic. You must cut out small pieces at a time or the plastic will melt behind the cut and seal it ...