... place a black garbage bag (which I keep in the rear trunk) over the plastic window, facing the sun.
In less than 5 minutes it will absorb and transfer enough heat to the plastic window to allow it to neatly fold even at temps in the 30s.
Here's a picture at one of the BRBSs when it was in the high 40s.
At home, when it's ever cold, I just place a hair dryer over the engine compartment which also quickly heats up the plastic.
Happy Boxstering,
Pedro