2 containers to use as molds (can be anything you want)
concrete mix (can be purchased at any home store)
Mix concrete together as directed.
Next use a paintbrush to thoroughly coat the outside of the smaller mold with cooking oil.
Fill the larger mold with concrete to about 1 inch from the top. Shake to eliminate air bubbles; level the surface.
Push the smaller mold into the center of the concrete, leaving an adequate thickness for the bottom.
Place a weight in the smaller mold and let the concrete set for 24 to 48 hours.
Gently remove the molds. Smooth rough and sharp edges with a stone or file.