Sorry guys... that hurts!
Yep, below 60 degrees is risky.
My car is stored top up and before I lower it for the season (my car is a sunny weather car so the top stays down most of the season except for washing and service) I close the car up and run it with the heater on full. Outside, just before I lower it, I go over the outside of the window with a blow dryer so it's nice, warm and pliable. Window is perfect, clear, flexible.
But, it's also now 12 yrs. old, so I'm fearing any day now...
Cheers!