I had this posted on the Arcus Google Group ,which by the way you should join as you get some good information there (Schempp-Hirth Arcus) including discussion on Canopy.

I had the same problem with my Duo Discus and a Blue Tinted Canopy .

I got a Canopy Cover made up from a Material similar to Hyperkewl http://www.techniche-intl.com/technologies/hyperkewl/
Powered by HyperKewl - TechNiche Intl.
EVAPORATIVE COOLING HyperKewl™ Evaporative Cooling material is a unique fabric that absorbs and slowly releases water through evaporation. We look at this process as “super-charging” the bodies natural cooling process, thereby protecting the wearer against heat stress.
www.techniche-intl.com
You soak this completely in water for a few minutes, then dry the inside and i used to place it over the normal grey canopy
cover as not to scratch the canopy as the inside was a plastic material. I used to place it on the glider in the morning before
the Sun started to Heat up. Even the outside Fabric Skin dried up very quickly the evaporative cooling worked for quite a few hours
because of the structure of the inside of this material. The closing the canopy before the flight was no longer a problem and the
Cockpit was also at an agreeable Temperature. However you had to be reasonably swift between taking off the cover and
closing the Canopy for Departure. You also needed somebody to take the cover away as you could not store it in the Cockpit.
Mine used to live in a Coolbox. So it worked well during my stays in the South of France (St. Auban) and Spain (Ocana) and Australia (Benalla)
After a few years the Canopy settled down and I gave the cover to Uli Schwenk of Jaxida Covers, as I thought he could be interested in
commercializing the product. No idea why this was not pursued as in my opinion it worked!!

Bruno