I am currently using a 40 watt reptile tank heater in the cockpit. About the size of a small bulb and fits a standard lighting socket. It is keeping the cockpit about 10 degrees centigrade higher than ambient and the relative humidity about 80% in a cold damp Scottish winter.

Others at my club are using similar power strip heaters and/or tiny electric dehumidifiers in the cockpit.

For many years previously I used a Ruby Dry dessicant electric dehumidifier in the trailer keep dry the whole glider but I was advised by Tilo Holighaus not to over-dry the canopy.