This is something I've been meaning to make for years, and have finally got around to doing. It's a jar for cleaning my airbrushes.
The jar itself is a big plastic peanut butter jar.
The black thing was designed in Blender and printed on my Ender 3. It has an inset ring in it that I've squished a bit of closed-cell foam into, and the foam has a hole cut into it that the head of the airbrush pushes into. It creates something of a seal around the airbrush head to minimize spray leakage.
I cut a matching hole in the lid of the jar and hot-glued the nozzle-sphincter to it, and drilled some little holes on the other side of the lid from it to release pressure while allowing a minimum of the spray to escape.
I can just load up the dirty airbrush with a solvent, jam it into the foam sphincter, and spray it through until it's clean. And there isn't a great cloud of aerosol solvent floating around my workroom.
Eventually, I'll have to empty it, but it will do quite a few rounds of cleaning before then.