My current favourite solution is to use an Artic F8 TC fan and zip tie / screw that in place of the original fan, then put the temperature sensor into an area that you know will get hot but not blown on by the fan, such as wedged in the gap between the heatsink & CPU if it's a SECC II cooler, being careful not to short anything.

The fan: https://www.arctic.ac/uk_en/arctic-f8-tc.html
Modern fans are so much quieter than the ball bearing / sleeve bearing fans of the mid 90s, and output more air in my experience.

Slotkets aren't much better, 60mm fans are still pretty loud without pushing a lot of air, but it is easier to fit a bigger heatsink into the space.

On one of my PCs, I used a plastic folder to make a duct from the slot 1 heatsink which goes to a case fan on the back, like Dell P4s used to do.

IBM, HP, Compaq, Gateway, All the big OEM companies had passive coolers, usually with a case fan or similar blowing direct on the heat sink.
Quick ebay search for Slot 1 P3 shows many examples, here is one.
https://www.ebay.co.uk/itm/IBM-00N3622-Pentiu … aEAAOSwAGlZ1Icw

It should be fairly straight forward to replace the heatsink, but its probably just as cheap as getting a CPU with the heatsink already attached as the heatsink by itself.

FYI, I upgraded to a socket 370, 650MHz Pentium III with my Slotket and a passive heat sink. It is quiet and stable so far.

Unfortunately the fans on the VooDoo 5 5500 are just as noisy as the old CPU fan. So I did not make much progress in my quest for tranquility. 🙁