this tool costs less than $5....score the plexiglass with the knife and straight-edge. You want to score the material on the side of the straight edge that you will not be keeping in case your line strays from the straight edge on a pass or two.
Continue to score the plexiglass until you are at least 1/2 way into the thickness of the material. Clamp the plexiglass to a piece of wood or a table and line-up the score line with the edge of the wood or table.....and snap. This should give you a clean line if done properly.