I use granite slabs ¾x 15x22 to isolate my speakers because my system is on the 2nd floor of my house. I got the granite from a Counter Top installation company. When installing the material they cut out for the built in ranges and sinks. The cut outs are left for trash. I gave the installation guys $20.00 and they cut it size for me. Masonry Drill Bits are available at most major Hardware stores.
Good luck.
Good luck.