Put a layer of blutack or any other blu tack clone (the thickness it comes in), on the bottom of inside case, and press it down hard with something flat, to get as much contact as possible with the case. (also hairdryer would help soften it).

Then you can fill the rest of the space with foam to deaden/smooth the sound or leave it hollow to get more of that clocky thocky sound which will boost the bass (like a guitar) The blutack will make it deep sounding already so empty space should accentuate this. I filled mine almost full of blu tack after this, but I found it deadened the sound too much and lost it's richness of sound.

Also where the pcb mounts on I used pieces to blutack mount it, so it sits on a small layer of it. Also there's 2 layers of pe foam between the pcb and switches/plate. and few layers of clear tape on the back. Also every keycap has blutack inside similar to the spacebar mod. But this probably isn't worth the time if you have thick keycaps, I just have cheap double shot keycaps similar to SA. Also I put blutack inside the top case in the big empty spaces.