I had a similar problem when I was flying a Ventus B. If help was
available, having someone push down on the nose distorted the fuselage
just enough to allow the canopy to close without too much trouble. I
tried the sanding route too with little improvement.
Between heat and humidity you can have some significant expansion in
materials. Most canopies are made from a material that is hygroscopic
and if you move your glider from a dry climate to a wet/humid one, you
can count on seeing and feeling the effects.
The forward hinged canopies seem to tolerate these effects better.
Respectfully,