Google woodworking cabinet scrapers. Used properly you can quickly shape the edges round with a beautiful smooth finish. Plexi seems to be made to be scrapped! It is a satisfying task with NO dust. I have done many canopies over the years with no cracks so far as I know. Try it on a scrap.
A good "scrapper" is the blade from a woodworking hand plane. Turn the cutting edge backwards, hold it 90 degrees to the work, and pull it towards you.. A bit of practice with the angles will give you awesome results.
This is the last step, after you get the part to the right shape. It is so fast that I take the sharp edges off the plexi each time I cut as I work towards the final shape. It makes handling the glass much safer for both your hands and the plexi itself.
Try it before you tell me it will not work better then hours of sanding.