Glider type and previous owner aside, I got a reply about how to do a
difficult sealing job on an older glass ship with a 6 foot long, one
piece, removeable canopy (Glasflugel BS1). It also has the problem of
the canopy becoming too long if it is in the sun and not installed on
the plane. Here is what I was told.

"This is an old problem, that only can be solved by making a gap on
the rear side of the canopy approx. 4mm when canopy is cool. This gap
is afterwards completely closed by adding a special mylar tape onto
the end over the top of the canopy bow. We call it V-band, it is made
by TESA company. Half of the width is covered with adhesive. This tape
does a perfect job, and the canopy is absolutely leak tight."

He sent me a small sample, that I have misplaced. The V-band is also
tapered slightly on one end to give a better blending into the
airstream. He said he has also used it on the front of his canopy
(stuck to the canopy, free end facing forward) and it works there and
does not lift and make a spoiler. Since your canopy is likely shorter
and doesn't have the expansion problem of the BS1, there is, of
course, no need to open up a 4 mm gap between the frame and fuselage!

I have also used just plane mylar on the back of the canopy on my Elfe
with good results. The drawback on the Elfe is there are pins on the
back of the canopy, and with the mylar on the canopy, you cannot see
the pins to get them to engage the holes on the fuselage.

Steve Leonard