My method which I've been using for a number of years with great results:
I cover the canopy with Bare Metal Foil.
Burnish it down using a Q-Tip
Using a brand spanking new #11 X-Acto I carefully score around the frame using just the weight of the knife.
Using the tip of the blade or tweezers remove foil from frame and I'm ready to paint.
Afterwards I remove the foil from the "glass" with a round toothpick. The point of the toothpick is sharp but won't mar the plastic any because the wood is softer than the styrene.
There are many other methods out there which others swear by and I've tried most of them. Many of them have worked well but I always come back to this one.