Rotocast by hand in quicksetting concrete using molds entirely cut on a Glowforge
I won’t go into a write up as the process was created and taught by another very talented designer, you can get an inkling of how it works from his public videos and posts. The molds are flexible, reusable and can produce an entirely unique form each time.
One thing I learned from the foam concrete folk was that they added aluminum powder to the mix and that reacted with the chemicals making tiny bubbles, that would increase the volume of the concrete, and be lighter. they then increased the setting hardness by steam heating keeping the bubbles where they grew. the result was very strong in total, but you could cut and sand it with a saw. and with a sealed surface freezing water would not hurt it and it insulated enough that you could make pottery ovens from it, without cracking or spalling, or losing energy as most other stuff does.
Phil Cuttance, the designer who developed this process, primarily uses Jesmonite - a gypsum and water based resin. I’ve seen other people use translucent casting resins. Since the molds are polypropylene they’re pretty non-stick to most casting materials - lots of room to play!