Some number of years ago, perhaps in about 2008, I bought several Presto Heat Dish units at Home Depot for about $20 each. They are electric 1500 watt heaters with a parabolic reflector. The reflector on those that I have are about 16" in diameter. Doing a quick search on google and Amazon, they are considerably more expensive now between $40 and $70. They also have more features than those that I have. These little heaters create a kind of heat cone that radiates from the dish. Closer is warmer and farther is cooler. The size of the cone at 15' is maybe 8' or 10' in diameter. Anything inside the cone and at perhaps a distance of 15' to 18' from the unit is toasty warm. Take one step outside of that 8' area and you are quickly reminded how cold it is. That $20 price point is a no brainer but getting 2 or 3 units at $70 is quite different in my mind.
When I was in Colorado and in the hanger on those 20dF nights and everything in the hanger was cold soaked and I needed to do a bit of engine work, I placed one unit 15' away from the engine area on the right side and another on the left side. Anytime that I was working anywhere near the engine compartment I was toasty warm wearing only a t-shirt. For me it was the perfect solution short of spending some real cash on insulation and a high BTU heater and vent system.
One caution however, if anyone chooses to try one of these parabolic type heaters, beware that the heat is very concentrated. If the unit is too close to you it will feel uncomfortably hot, If the unit is heating your work area as in my example, it is also warming some of the airframe just rearward of the cowl. If the unit is too close, the sides of the airframe can get pretty hot and maybe even cook the paint right off. Just be sure to keep the unit far enough away to prevent any heat damage. Even though they are only 1500 watts, the heated area is very concentrated.
Dave