gel/foam/rubber pads?

ummm i'm no expert on this but ... at my stable they put a black pad made out of foam (not sure what it's called) about an inch and a half thick between the numnah and the saddle. it's shaped to fit right into the bottom of the saddle, up into that groove.
 
i use an impakt

pad with my horse ,just as an extra cushion because he is very old and we go over rough terrain and i just want to be sure hes comfy i think they start from £20 i've had mine 2yrs now and i rarely ride without it!
 
newrider.com