Doing a bit of research, I found that PU could be used for nice phantoms, having acoustic impedance close to that of water.

I tried two types (hard and soft, respectively PX90 and PX30) - and casted them into a silicon mold for financiers.

Tried different layers of hard/soft, including some coffee grounds for diffusers.. curious to see that these will yield.

Really curious to see what they'll give.