This only cost me about $8 to build.
I bought a piece of 12"12" slate from Home Depot, measured out the size of the pieces that I wanted. Cut them and then used aquarium glue to glue it all together.

I bought a piece of 12"12" slate from Home Depot, measured out the size of the pieces that I wanted. Cut them and then used aquarium glue to glue it all together.