I always wanted to have one, of course :). But the ones I found were quite expensive, and that is what usually puts me to think (there's a saying which states something like "the need makes the inventor" ;)).
So I looked at my small material stock and this squared section 1.2x1.2cm (1/2 inch), 100cm (40 inches) long piece of wood draw my attention. I picked Inkscape (excellent OSS 2D vectorial drawing application) and designed this marking gauge using only this wood stick, a nail and white glue (also known as "glue for wood").
Before starting with the construction, I did a paper model of the headstock, just to have a feeling if the measures I choose would adapt well to the hand. They did.