On Monday, January 7, 2019 at 6:10:15 PM UTC-5, Duster wrote:
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.

I wouldn't suggest trying to force it closed.
Getting it wrong is about an $8000 mistake.
To find out where the interference is try slipping a narrow(about 1/2 inch wide) strip of paper between the canopy and the frame to find out where the interference is. It may not be where you think. That would provide some sense of where an experienced person might do a little sanding.
Been there- done that
Good luck
UH