I fly as a guest pilot in a friend's Arcus T which has a similar, and quite severe, issue. Closes fine in the hangar, but once the sun hits it the canopy will not close. What always works is draping wet cloths over it before it gets hot. Seems fine after that, even back on the ground for awhile (caution; remove towels before flight!). If the frame-sanding option is ruled out, one thing you might seek expert council (Mr. Nadler?) on is to increase the bevel of the bayonet locking bars where they engage the receiver pins on the gunwale. Factory bevel is about 45deg and may not provide enough leverage force to mate with the oval pin opening. Not sure if grinding the bayonet is kosher. The ones on the Grob 103 have a long, more shallow taper. The factory has not been real helpful, unfortunately. Hmmm, yet it shares the same fuselage as the Duo Discus.