I have created a single line font based on circles so I will call it a strict geometric font. There are two font variations – one more rounded than the other. You are welcome to download and use my fonts under the terms of the SIL Open Font License. The zip file comes with the two svg-fonts to be used with the Hershey Text extension as true single line fonts. In addition I have provided the corresponding stickfonts as .otf (and .ttf, but I recommend the .otf). The stickfonts are not true single line, they will be drawn twize with your plotter if used directly without converting to single line. See this post where I explain how to use the svg-fonts.
Other single line freebies