Some info on harmonics and the guqin:
You should be able to take any string (say on a guitar) and practice hearing the harmonics. What you do is touch your finger lightly on string, then pluck the string and hear the harmonic. The one at the octave is the easiest to hear.
What I have done to copy instruments is to use a straight on photo, like the one above (the bigger the better). Then blow it up to full scale in drafting software. Then trace over it. You will need to know the scale length to get the measurement exact. Then you can print out the drawing full scale and build your instrument from that.
So by combining all of the above, I think that will get you there. You can use the harmonic information or sounds to verify the drawing and vice versa.
Thanks for that helpful info, Mike
Wikipedia has a fair amount of help here: