Figure 1: A Lucet
A lucet, sometimes called a chain fork, Strikkegaffel, Knyttegaffel, Snoregaffel, or Nullestok, is simply a tool with two arms that helps you knit cord. Figure 1, above, is the basic shape I make and use. Some lucets are short, without any handle. The one shown in the Ashley Book of Knots, #2875, is almost heart shaped. Some have a hole drilled below "U" that the finished cord is threaded through, but I've never seen the reason for that.
You can find examples with many shapes, made of metal, bone, wood, ivory, etc.   They all work the same.
Nothing says the "U" has to have a round bottom. A square or a "V" works just a well, since it's not involved with the actual work.
A coping saw does a good job of cutting the shape, but you can make a perfectly good lucet with just a pocket knife.
Make your first lucet with the tools you are most comfortable with. You'll have plenty of opportunity to change your technique when you make your second, and third ... lucets. They are easy to lose, and fun to give away to new cord makers. And each new lucet lets you fix the things you didn't like about the last one you made.