The best way to do it is to basically make a frame for each section of railing between the posts. Cut a groove in 2x2's that is just a little wider then the thickness of the lexan that you are using, and make sure that you cut the groove 3/4" deep. Then miter the ends of the 2x2 to make the frame to fit between the posts. After you have mitered the frames, put 3 sides together leaving the forth one off so that you can "slip" the lexan inside the frame. measure the inside dimensions of the frame. Add 1.125" to both of these dimensions, and cut the lexan to these dimensions...... then slip the lexan in and finish making the frame by attaching the 4th side of the frame to the rest of it. Then mount the frame to your posts and enjoy your view.
