Mark your cut lines with a water-soluble whiteboard marker and a flexible straight edge. Stray marks with this marker can be easily wiped off. The edge of vinyl electric tape can also be used to mark your cut lines.
Make sure all the bottom edges of the canopy are supported when cutting. Make sure the plastic is not bent or under any kind of stress while cutting.
Use an abrasive cutoff wheel. A wheel made to cut steel works great. Make sure the wheel is balanced. One wants smooth cutting. I use an angle grinder and a 5-inch wheel. A dremel tool or air tool also works well, just not as fast.
Don't use a jigsaw. Don't use any kind of tool that isn't smooth.
Test your drill bit on a scrap piece of plastic before you use it on the canopy. See if the bit grabs as it exits the hole. If it does of course don't use it. Don't press hard while drilling.
Sometimes a canopy has to be bent to get it to fit its frame. In this case, bend the canopy, mark where you need to drill, let the canopy relax, then drill the hole. Make sure the holes are larger than the rivet, screw, or bolt you are putting through it. How much larger? Enough so nothing binds.
Use products that are made for cleaning plastic. I use Novus products. Don't use household and glass cleaners. They may and probably do contain chemicals that attack the plastic. If there is a lot of dust on the canopy, spraying compressed air on it works good to get most of it off.
Todd
(Courtesy of George Balmer)