sometimes black mousepad foam can work (you'll also need double sided tape). I used that on an old 80's SR4 I had a while ago. Just cut it into small squares and you are good to go.
This may not accomplish exactly what you are striving for, but I utilize a small piece of felt at the first fret. It is about half the width of the first fret and it slides up next to the nut. It does a pretty good job of mutting, it stays in place and it's easy to insert and remove. You can get the felt at most hobby and fabric stores.