The typical concrete mix is 1-2-3, 1 part cement, 2 parts sand, 3 parts stone. In your case, 2 parts Portland cement, 2 parts builders sand, 1 part small small stone. also a handful of long fiber fiberglass. Mix well. Water is the most critical part. Start small and slowly add until you get a bread dough consistency. In a water proof mold, you may need to have a very dry mix. Start small and experiment. The basic concrete mix is not what you want here.
