...how hard is working with upholstery foam? Like, cutting it? Does anyone know?

found this: Secret Lock Book by mrhers2 - Thingiverse

thinking bout making it, but lining the inside with upholstery foam to keep stuff stable and not make it sound like things are rattling around.

I think you use a electric knife on it to keep it from making you crazy?

I want to say that a heated blade would work, but I dunno about flammability or fumes. @_@

Hot wire maybe, but only for bulk cutting. I don't ever remember seeing it with melted edges, though.