the cone is something I install on all concrete in posts. It just creates a sloping design that allows water to run off and away from the actual hole. I do not think that you could fashion anything with gravel to accomplish this effect, due to the channeling into the hole through the gaps in the rocks.
I see many fence jobs where no concrete was ever used in the holes. I shutter at this and would NEVER do this for my fence or anyother customers either. Maybe this was intentional or the contractor cutting costs and making more profit on something he or she should have done anyway.
In our windy area, the fence just would not stand. But, if your not windy and the hole size was matched somehow to the size of the post, then it might just work.