I've seen wavy cutting wires made from stretched springs, but this one was made by separating out a single wire from a steel cable, the likes of which you can buy by the foot at the hardware store. Hat tip to my dear friend Ginette, who is also my student but could just as easily be my instructor. The steel-cable method produces a cutting wire with a shallower wave.
I paint a bit of slurry on the rim, so I can use it to demark where to cut the textured slab which will be the top. After attaching and smoothing, I made a one-inch cut on the narrow side, an inch or so down from the top. This is so, when I cut off the top (to become the lid), the resistance to the wire doesn't deform the piece.
I rolled a small coil - smaller in diameter than my pinkie finger - then flattened it, and attached it to the inner edge of the body of the pot, to stabilize the lid.
Still needs a bit of spiffying up with a paddle and rib, but viola, more or less.