Having never cast concrete, I’ve added a draft angle and rounded hard edges on the ashtray for hopefully easy release. I also added a raised lip to the mold so it can be tapped on the ground so gravity can help with mold release. It also should help the concrete pour from leaking and allow you to scrape the surface flat with a trowel or similar tool so the bottom is flat. I also added an overhang to the side so when levelling the concrete, you can get a second container close to collect the overflow.

Can I improve this design? Any suggestions for other molds? Will upload STL if there’s any interest and after any suggested improvements have been made.