I printed the Simple Dice Tray from Urici and wanted to make a noise dampening mat for the bottom. I then realized that I could make the same for the cover of the box, plus adding a place to keep your dice.

I made one from TPU and the other NinjaFlex. Both work quite nice but I like the way NinjaFlex sounds more.

The lid side mat STL also fits my DM screen/dice box found here: https://www.thingiverse.com/thing:3705297