Franklin Fabrication sells a fitted canopy cover for just this
purpose.
It is made of Mylar coated stretch fabric and secures without straps
via elastic gathering like a shower cap. Very good heat and sunlight
reflection. Quick removal on the grid and easy storage in the
glider.
Cheryl @ 760) 715-6930 or
She has patterns for many sailplanes and I think she is working on
one for the Ventus and Discus.