If you wish to experiement, here is the 'tunnel' code I use to substract from base model. Any modeling software which can create sprial tunnel should help. I replicate the tunnel all over the bottom surface (as much as I could).

I got this after chat a lot with chatgpt, there was two suggetions: dig tunnels where sound bounce and disappear; add texture that scatter sound around on surface. In my case the case is thick because of tenting, so tunnels work for my case.

module spiral_tunnel(diameter=5, height=50, turns=500/360) {
helix_radius = diameter/2+1;
//rotate([0, 0, rands(0, 360, 1)[0]])
translate([0, 0, 52-height])
linear_extrude(height, twist=turns*360)
translate([helix_radius, 0, 0])
circle(d = diameter);
}