I made a heating element from a surplus coffee warmer wrap that I found at the surplus store. It was a long, flexible winding of nichrome wire wrapped with a braided fiberglass cover. I unwound the nichrome wire and measured the resistance. It turned out about 1 ohm per inch. By measuring off a few inches and putting in several strands, I was able to come up with the right resistance and the right length. I buried the strands in casting rubber and let them set up. My wife made a nylon cover with velcro fasteners to hold it on my refractor.
It works fine... Cost was minimal. Most of the expense was in the rubber, of all things. I might do that part differently on the next element I make - maybe a silicone rubber wrapper.
Bob