Clean your Plexiglas regularly with cleaner made specifically for acrylic. This will keep the Plexiglas from exhibiting crazing as long as it is kept shaded.
Cut a piece of canvas 6 inches longer than you would like the finished sunshade to be. Place the PVC pipes at the ends of the piece of canvas and wrap the canvas around them. Leave a small amount of extra room so that the pipes can be slipped in and out. Pin the canvas around the PVC pipes with sewing needles.
Remove the PVC pipes from the piece of canvas and sew the ends closed, leaving the area where the pipes fit open. Slip the pipes back into the ends and place the sunshade over your Plexiglas to protect it from crazing.