Materials used for these are as cheap as possible: CDs, paper, sand, decoration stones and Creall Do & Dry modelling material. First, paper was glued over the CD to cover the obvious hole in middle and to work as a more adhesive surface for the Creall. After that, deco stones were glued onto the surface; while this is not required, it gives the terrain more character and more support for the modelling material.
My girlfriend took the idea bit further, and actually made her crater look good. In fact, she hasn't made a shell crater, but a small volcano - and she did a pretty good job with both the modelling and painting part!
The lava was made with PVA glue - first it was poured into the crater to make level of certain height, then (after waiting a few days), the bubbles were made on top of it using more glue. I'm using the volcano as a mysterious lake in game, though the size is limiting. But it looks great on the tabletop!