It's remarkable at altitude how the cockpit temperature goes from being comfortably (or even uncomfortably) warm in sunshine to
cold in cloud shadow. Electric insoles inside moonboots are very effective, a 1 minute burst every 15 or so keeps feet warm.
The coldest glider I found was the SHK, with quite a small canopy for sun-warmth and a very thin non-structural fibreglass nose. That was a foot-freezer at 18,000ft. Thermal underwear with layers are (as has been said earlier) the way to go.
As a lifetime non-smoker I have always been OK up to 14,000ft or so without supplementary oxygen, and higher on short rapid climb and descent; flying in Spouth Africa all day at 10K to 16K an EDS system means I don't get a headache after flying (and increases the alcohol liimit).