It's a regular terracotta/clay flowerpot. I created the "portal" by carefully applying the old hammer, not too brutally, and lucked out big time in the first try ! The "portal" was perfect ! Next I grinded the rough edges down with sandpaper so Edgar won't cut himself. This took 5 mins tops. The other pots were simply cut in half with a regular carpenters saw. These didn't require sandpaper smoothening because the edges are embedded in the substrate, and there are no sharp edges for the animal to hurt itself with.
Good luck !