I keep a mold handy for using up left over resin. This is my latest. The black resin I had left over didn't quite fill the mold, so I cured it at an angle, then when I had more of another color I poured it on top.
I've 3d printed out a variety of thingies that would make good molds, but I've only made a mold of a star knob for jigs and such. In the shop at work we pour the excess resin into cracks in the floor .