https://www.ammoland.com/2019/08/diy...#axzz5xkSB1vSI
Basically add some extra foam inside to fill in the empty space, and replace the ear cups with something that seals, conforms and blocks sound better.
They suggest ones from NoiseFighters, but I've been using these Valhall Gear gel cups from Amazon:
https://smile.amazon.com/gp/product/B0719D35HM/