For a string that short, you really don't need a full board--a board with two nails in it will work just fine.

Figure out how much you want to twist back into the string to form the loop--I like around 8". You want a finished string 26" long. 26"+16"=42". Divide by 2=21". Drive two nails in a board 21" apart, wrap the strands and cut. After you build the loop and get down toward the end of the bundle, start dropping a strand or two every twist to taper it off.