Mine is in a twin tier slate plinth. Try picking up some slabs of slate from a stonemason and either cut them out yourself or find a local place with CNC facilities that will do it for you. I’ve heard 401’s in various plinths and slate sounds best to my ears.
“Music has always been a matter of energy to me, a question of fuel. Sentimental people call it inspiration, but what they really mean is fuel. I have always needed fuel. I am a serious consumer. On some nights I still believe that a car with the gas needle on empty can run about fifty more miles if you have the right music very loud on the radio”
Hunter S Thompson