paint the top.
Why do you think gliders etc are painted white? to reflect the sun / reduce heat absorption.
It does obviously depend on the resin used, the a-typical epoxy you get online or from the local composites store will have a low Tg, sometimes as low as 60deg C but with proper post cure and resin selection you get get the Tg over 130deg C. You can also get some resins which you can post cure at 60-80deg C but will inservice post cure and not distort until it reaches a much higher temp.
I use Hunstman 5052 resin system which with an 80deg cure raises the Tg onset to around 110deg but with a 100deg cure can be raised to 130+deg.
Last week in the UK with only a 20deg air temp and blinding sun I had to take the board inside as I couldn't touch it to work on it!