I've never tried the pre-cut masks, but I usually get fantastic results using Bare Metal Foil to mask the clear areas.
First, I dip the canopies in future and let them dry for AT LEAST 48 hours.
Then I cover the canopy with the foil, using a toothpick to make sure the foil gets into the recesses and corners, use a new no.11 blade to trim the frame, then peel off the foil from the frames. Make sure you use the toothpick to re-burnish the foil on the cut edges to prevent the paint from seeping under.
Then paint the canopy the INSIDE frame color, THEN the outside frame color - that way you can see the inside color if you look through the canopy. When the paint is dry, LIGHTLY score the edges where the foil touches the frame, and then ease the foil off with a toothpick. There's usually not much adhesive left behind, but if there is I use a q-tip to rub it off.
Works for me - hope it does for you, too.
Cheers
Lee Tree