I try not to let Mother Nature's icy cold keep me from building during the winter. So when it's cold outside, I use a heat tent to cure parts overnight. All epoxies have a recommended minimum cure temperature from the manufacturer. You'll get the best results if your parts are cured above this temperature within the first 12-24 hours.
A heat tent is nothing more than a tarpoline or some plastic suspended over a temporary frame. You put a small electric space heater underneath one end of the tent. The parts to be cured are placed at the other end. Heat tents can be made from whatever you have on hand. Saw horses make great frames. I have cured small parts under a large cardboard shipping box. I've even cured parts in the fuselage with the canopy closed! Heat tents don't need to be fancy. Use whatever works to hold the heat in.
In the photos above, I suspended two 5-foot aluminum angles over two of my wing cradles. The heater is on one end and my composite roll bars are at the other. I use bricks to hold the plastic to the ground. I make every effort to seal off openings to keep the heat inside. I have a bucket of clothes pins that come in handy for cinching larger sections of plastic when it's too impractical to use bricks. It's not important that the heat tent be hermetically sealed. It just needs to be free of open holes and gaps that would let the heat escape. I don't like to use duct tape or masking tape because it will come loose once the plastic heats up and the glue on the tape lets go.
The heater I use is a 1500 watt electric space heater available at any Lowes or Home Depot. It is a convective heater, which means that it generates its heat by blowing air over a set of electric coils. It has a thermostat. It also has a safety switch that shuts the heater off if it falls over for some reason. Because the heater has a fan, the heater will circulate the air within the tent. I normally set the thermostat at around 85-95 degrees. While the heater can produce higher temperatures than that, I like to stay at 85-95 degrees. That way the heater will cycle on and off throughout the night. It will run for a minute or so, then shut down for a few minutes. Any higher than that and the heater will run all night.
For safety reasons, I try to set up the tent over the concrete floor and away from convenient flammable materials. I try to ensure the plastic is at least 3 feet away from the heater and that it is taut and cannot be sucked up against the heater. Sometimes it's impossible to keep the 3-foot rule. That's another reason I like the convective heaters. At 85-95 degrees the heater coils do not get very hot. There are very few materials that will combust at that temperature.
I stayed away from using radiant heaters. They generate their heat with wire or tubes that glow red hot. Most of them do not have fans, so they are useless for circulating warm air under the tent. The radiant light can melt parts if placed too close. Plus, "red hot" to me means a greater chance of FIRE. And for goodness sake, DO NOT EVER use a propane, oil, kerosene, or any heater that uses an open flame.
Need convincing of the benefits? These pictures were taken on January 23, 2005. It was 25 degrees in the hangar and the wind was gusting to 30 knots. Yet inside the heat tent, my composite roll bars were curing in a warm, tropical paradise of 90 degrees. I returned the next day to find my parts cured rock hard.