ORIGINAL: saramos
I would recommend leaving A Lot of extra balsa, cut out the canopy, dry fit it, and draw a line around it. Then cut, say, a quarter of an inch inside the the line and trim it to fit when you finally are ready to mount the canopy.
that's pretty much how i did it. i would suggest not cutting the canopy out completely; leave, say, 1/8" to final trim later. other than that, ditto.