We use a similar set-up for sampling geotechnical monitoring wells, plastic bailer inside a deep well. When we occasionally lose a bailer down a well, we use a treble fishing hook on the end of some strong fishing line. If you happen to live near an area where salmon snaggers "fish" you may be able to get a weighted version of a treble hook. Doesn't get the prize every time, but it has a good track record for saves. Just run the hook down the well & jig it around some & it should snag some of the line on top of the PVC tube. Of course, the treble hook also snags anything else that may be in the well, too.