What is Reddit's opinion of Klein Tools 53716 Flex Bit Auger with Screw Point, 3/8-Inch x 54-Inch Long Flexible Drill Bit / Fish Bit for Pulling Wire Behind Walls?

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>suppose I could fish it onto my bottom floor and then ask someone to listen to the location from inside the wall?

That's the way I had to do one of my walls. I couldn't get a good reference point in the attic. Drilled the top plate, pushed the rod all the way down to the bottom plate. I asked my wife to listen as I tapped the bottom plate with the rod. She put a post-it note on the wall where she heard the tapping. It was close enough.

There's this:

https://www.amazon.co.uk/Klein-Tools-53716-8-Inch-54-Inch/dp/B007E764ES

and this:

https://www.amazon.co.uk/Amtech-F0770-Flat-Extension-3-Piece/dp/B00593JZWC

Not sure about shipping times, sinc eit looks like Amazon computes shipping times to my US address.

You might need two of the latter, if you have enough room to work. Make sure the set screws are tight. If it comes apart in the wall, it would be hard to recover.