so yes, anyone do this? basically the saddle on a friend’s bike keeps moving backwards in the clamp regardless of how tight she clamps it in, and she’s talking about putting shims in it to stop it moving around.
what do/would you use for such a thing?

bit of coke can kept the bars tight in the cinelli stem on my old roadie, did about 5000 miles on it without movement. Think it was a 25.8 bar in a 26.0 stem so wasnt a massive pack.

Tighten it up more. Using a ‘proper’ allen key, not a multitool. Ie something like this with plenty of leverage:

coke can shim/tin foil will work as you only need very thin metal to do this