Sound Addicted - Studio Monitor Isolation Pads for 5 Inch Monitors, Pair of Two High Density Acoustic Foam which Fits Most Speaker Stands | SMPad 5

The best thing I found was bookshelf speaker foam things. https://www.amazon.com/dp/B0757LTZJJ

It's definitely expensive but I did an A/B test for my bookshelf speakers and I did much prefer them on the stands. Isolating the speaker from the surface is good plus you have the ability to angle the speaker to the listening position. You can also find much cheaper options to angle the centre speaker https://a.co/d/6Or2wOC