If you have a welder outlet, try one of these; 5000 watts and 18,500 BTU :
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1.
I have one in my insulated 1200 sq ft shop in Minnesota and it works great. I usually winter in Florida, so I only bought it to ward off the chill in the spring and fall. I had to spend the winter of 2017/2018 in Minnesota, and was able to keep it at over 60 all winter (electricity is included in my maintenance fee). The best $130 I ever spent.