I use a cut off from an extra-thick Minky Sponge as it is just the right height to fit under my Jazz bass strings. But you can use anything, didn't Tony Levin use a nappy during the recording of Don't Give Up with Peter Gabriel.
https://www.amazon.co.uk/Extra-Thick-Sponge-Wipes/dp/B009R0HI86/ref=sr_1_1?dchild=1&keywords=extra+thick+minky+sponge&qid=1620504618&sr=8-1