Until very recently, these were disks of wood, usually with a cross bar fastened to the top side to act as a handle, that loosely fitted into a cooking vessel. Those are still available, but tend to be overpriced and only fit one size pot.
This stainless steel version can can expand from 5 inches to 9 inches (13 to 23 cm). The central pivot is easily released by squeezing the retaining clip so it can be spread all the way out for cleaning. If it sinks below the surface, it can be easily hooked out by the central clip. This one cost 2022 US $20.
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I was mystified as to where I got this, I certainly didn't buy it. It
is so large it would only work as an Otoshi-buta with my 5 quart sauté
pan and 20 quart stock pot. I finally realized it probably was a lid that
came with a Japanese cast iron Wok I was given by the sister of an unstable
friend who finally did himself in. Woks like that one now sell for over
2022 US $200. I do not use that Wok because it's heavy, a little small,
and doesn't respond quickly to changes in heat. Chinese cast iron woks are
very thin, and very fragile, but much more responsive.