Does anyone here have experience making gemshorns? I am trying to learn how and I have some questions about plug size, and how large the labium opening should be.
I started … and failed miserably 
However, one concept I did learn, practice with a suitable diameter piece of plastic pipe. This gives opportunity to experiment with labium sizes and angles without risking what might have been an expensive horn.
As I’m sure you’re aware, a gemshorn is basically an early ocarina, so the final pitch of the instrument is relative to the internal volume of the horn. Best start a bit too large and bring it up to pitch by filling it a bit with beeswax or similar.
Good luck 
Gemshorn for sale, made from cowhorn. Tonic note is E and scale is pretty accurately an E major (according to my electronic tuner); a bit longer than 13". Wooden plug fitted, with channel to create fipple. Quiet, breathy sound. I am the original owner (not maker) and haven’t played it for over 30 years, so it’s time for it to find a new home. Ask for details and a photo or (if I can manage the I-fone thing) a short video. Send private E-mail to gajdas_cp@verizon.net. Instrument located in Maryland, USA