FWIW if using a spring washer (see above) it is useful to use a flat washer on the neck.

- image001-Small Parts 316 Stainless Steel Flat Washer_SL1200_.jpg (15.66 KiB) Viewed 1017 times
https://www.amazon.com/gp/product/B009OK0976/
This not only protects the neck stump, but gives the washer something to bite (other than the TPE).
The spring bites the metal washer, which then grips the TPE and reduces rotation.
Another option is to use a spring. Insert it over the threaded connector to prevent turning.

- image002-Spring Steel Coil Compressed Spring.jpg (17.47 KiB) Viewed 1017 times
https://www.amazon.com/gp/product/B07MB8TWDM/
When using a spring it will allow you to rotate the head to a desired position, then hold it in place.
The head will not be "crank down tight", rather more difficult to rotate. But not flopping.
Using a washer under the spring will also help protect the neck stump TPE.
The 1.2mmx20mmx25mm spring (20mm diameter, 25mm length) seems to work best.
Cheers!
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