This is a modified version of the standard 19.5 mm ground glass spacer that comes with the Lomograflok Instax Wide Film Back. This should allow you to achieve sharp focus at wide apertures with a Speed Graphic that has a Fresnel focusing screen. I have tested this at f2.5 with a 178mm Aero Ektar lens and able to achieve sharp focus. The spacer thickness is 21.5 mm.
I know it works for my Instax Wide Film Back, but I have not tested it on other backs.
Recommended Print Settings: 0.3 mm layer height with a 0.4 mm nozzle. Infill 15%. Brim necessary.
The author hasn't provided the model origin yet.