I've done parts that way, and if you're careful, it will work. You can make hatch flanges on parts that didn't have them, by using strips about 1/2" wide of 1/64th plywood, epoxied to the inside surface of your canopy. (1/4" glueing surface and 1/4" flange) Just be careful not to distort your canopy part by inducing stresses with the flange. (I recommend pre-bending the canopy frame material before installing it, so it's just a plain glue joint and not a "stress causer".)

If you're just making one part, all of this is ok. If you plan on repeating it, you might consider the more tedious, but more precise and repeatable method of making male plugs and female molds, with your frames and flanges already built in.

Let us see your results here when you're done!