Here is a rig to hold the guitar while pounding in frets 14 – 19. Jose Romanillos surrounded by students. Siquenza, Spain 2004
(Click for a clearer larger picture.)
The piece of wood that is inserted into the guitar sound-hole is a heavy J-shaped unit, shaped from a single piece of wood, which is then suspended from the rig you see here….
I’m looking on in the background, skeptically, as Jose starts begins.
I believe these pics were taken by Geert VanOpstal