We have a custom concrete shop that does countertops and other such things... when I approached them, they showed me their Boneyard where I found two closely matching rejects... they cut them to size for me, and drilled some holes. I epoxied the threaded inserts into the holes.
I'm leaving now but will post a link to that other photo later when I have the chance.
Another option is granite, if you have a source near you. Usually some kind of outdoor landscape and quarry type shop. we have one in Santa Rosa called Weeler Zamboni that will do custom granite to your spec. It was like 70-80 for the granite. The concrete shop charged me 50 for 4 pieces. The wood cutting boards are the least expensive.
The outriggers are pricey, but that was my bling since I shopped smart on my speaker purchases!