It depends on the amount of plexi, and the size , but you can cut it by taking your regular saw blade and turning it around so it is turning backwards. Take your time witrh it .. Try it on a little scrap to see how well it works for you .. I trimmed a few sheets of 1/8" plexi using that trick .
You can cut plexi with a box cutter. Maksing tape off the area to be cut! Draw the cutting line on the masking tape. Cut a good solid straight edge on the line. Score the plexi along that line. Then place that line on an edge and push down. The plexi will snap along that line.