I use scissors & blades to cut vac-formed canopies. I usually cut very close to the frame w/ a pair of scissors, then trim closer by using a single edge razor blade & jewelers hammer on a hard flat surface (I have a tempered glass surface on my workbench that works quite well).
Start by cutting the excess plastic away w/ scissors. Next, place the edge of the blade along edge of frame on the hard surface and tap on it w/ the hammer to cut downward (guillotine style). I then separate the front windshield from the sliding part by using a SHARP #11 blade or razor blade to slowly cut them apart. If you can start the separation on both sides, you can use a razor blade by holding the canopy inverted, & SLOWLY work both sides at the same time. This will keep the edge even & square on both sides of the canopy. I sometimes separate the front from rear before I make any other cuts, depending on the canopy shape or size. Finish off by using fine sandpaper along the edges to smooth them. A quick dip in Future males them ready for paint.
I have done it this way for quite some time now, and the process is pretty easy after you get the hang of it .The key to the whole process is PATIENCE. A quality vac-form is also a plus, making it easier to see the separation lines to cut along.
I'm sure someone out there has a better method, but this is what works for me.