I made some craters out of clay. They took about 5-10 minutes each. The only problem I had was they tend to warp while drying, so I had to sand them down a bit on the bottom. They still wobble a bit so I'm going to patch greenstuff over them to fix that, and to add some asymmetry to them. The double crater developed a crack so I have to patch that up too. After I'm done fixing them, I'm going to paint them to match my table.
I also finished up the bases for my ruined temple heads (like Easter Island heads), but I have to do some battle damage effects before I can glue those on.
(Note that this is not Sculpey or a similar air-dry clay, this is a stoneware clay body, the same stuff used to make pottery.)
Here is how I made them: I took my clay and rolled it out into a slab about 1" thick. I then pressed a curved plaster mold into the clay. With the mold in place I used my thumb to push down the outside into the outward slope. To make them uneven I used my fingers to stretch them out a bit. Finally, I used a loop tool to scrape down the sides to give it texture. After they dried, I sanded the bottoms and fired them. I assume you can make these using air dry clay, they'll just be a little more fragile.