First, I recommend using a small nestbox for wintering. It only needs one small entrance hole for the animals and no vent holes. Their bodies will get it very humid and warm inside and that is just great. A small catfood or litter container or other plastic thing with a top access door is perfect. You can buy a cheap wireless or probe thermometer and keep tabs of the inside temperature to see if you need to do more.
Second, be very careful of adding heat to any animal situation. Nothing really exists on the market to do this correctly with small furries. High voltage and no temperature control can kill your cage kept animals fast.
http://www.sugarglider.com/gliderpedia/index.asp?Heating
I have a design in my head for a wooden nestbox with a false wall inside that allows you to protect a 4 watt heating pad in the wall. You absolutely must keep the high voltage devices and cords away from your little glider teeth and you must also not allow for direct contact with the heating pad.
Also, never ever use anything to warm the animals unless you have a temperature probe inside where the animals will be that will turn off the device when temps reach 70 to 80f. They do not need anything more. So be sure to have a thermostat on the nestbox.
There are a few small portable thermoelectric devices on the market that might be a good warming/cooling nestbox but almost none of them have temperature controls, they just cool constantly or heat constantly. There is one on Ebay that has a digital setpoint control and might be a neat in-cage warmer/cooler. You just set the temp to 75 and it will maintain it both directions. Cut a hole in the side for a doorway and you are good for all seasons.
I built such a thing to moderate temperature for my larger furry animals that I kept outdoors or in the garage but I have never had the need to maintain my gliders this way.
Robin gets a heater
The best heating for a glider is a colony of gliders in a tight box.
I have large nestboxes in my cages that keep the families nice and warm through winter. These are extra tall on purpose though so that I can have a top-down
webcam
in them.