Designed in the hope of making my practice quieter. The sound hole of my guitar measures 84mm across, and this is designed to fit snugly between the edges. My guitar is an Epiphone c40, so this should definitely work for others of that model, but may also work for other guitars with the same sound hole diameter. (or may be able to be scaled to fit)
I used Ninjaflex, but any flexible filament should work. 2 walls, 2 bottom layers, 4 top layers.