You can make these without much trouble. The mixture is called hypertufa. It’s basically a light concrete, roughly equal parts Portland cement (for cohesion), coir or sphagnum moss (for texture), and perlite or vermiculite (for volume without weight).  Mix with water and pour into greased containers.

They have to cure for a few days. Then they have to be rinsed for a couple of weeks to let the lime leach out.

Rachel, Veronica, and Shengnan made these. I made them wear dust masks and gloves when handling the cement.

They do good work.