Not sure if someone has a better mousetrap, but I've been using Styrene (plastic) to frame up my canopy, which I am custom making into a sliding one. What is really great about this stuff is that the solvent used to weld this material also works on the canopy material. Cut the Styrene to shape, hold in place, add a couple drops of the solvent, and after 10 seconds, it is a solid piece. There is a slight fogging of the material where it is dissolved and re-formed (right at the joint)- if you can't cover this up with paint or tape on the outside, this may be an issue.

Depending on the orientation you want to build your frame in, you can cut out a shape from flat stock, or you can heat up and bend strips or square stock. I'm really having fun with this stuff.