Keeping warm in a glider
Best way to keep warm in a glider in the winter is to fly the tug instead!
(which has a nice heater)....
But seriously, I did the altitude flying at Aboyne over 3 seasons....and never got that uncomfortable, didn't stay up all that long, and was lucky in having sunshine available. Though flying back and forth along the higher wave bar, found it interesting that one side of the glider canopy iced up completely, the other side was getting all the sun so viz was fine on the left side but rather doubtful on the right. A banana in the cockpit pocket (lunch) froze solid.
The year before, was swanning about at 15,000 or so, and because the glider had stayed out overnight and the wingtape got wet, the ailerons wouldn't move, they were frozen. So did very careful turns with the rudder alone, eventually descending to warmer levels.
The diamond gain of height to 20,300 was enough of the upper levels for me, I get nervous having to depend on oxygen systems. For cross country flying, especially in competitions, one tends to work the height band of thermals between 2,000' and 5,000', using cloud streets if possible. Temperatures are quite comfortable at these levels....and the baby blanket is handy if it does get chilly. Flying in the winter is OK especially if there is sunshine.
But standing around on a windy airfield in winter can get very very cold if you are not properly dressed.