If you’re just looking for something quick and dirty, the steel wire handles of steel, black, paper binder clips can be removed from the spring clamp itself, and they spring “outward”, as it were, and have a convenient loop to fasten a probe.
They come in sizes from 3 inches down to 1/2”, and are available at pretty much any stationery store.
You could remove the handle from the clip, compress the ends, put it down the hole, and the legs will bear against the side of the hole. Cheap and quick.