Agroshield
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....Going with 4 parts washed plastering sand, 1 part white cement and 1/2 hydrated lime.
On the section through the mould you drew, if the part between the bead and the edge is covered over, it will trap air, no matter how much excitation you give it. If you can suspend the bead another way, such that the full perimeter is exposed to the air, you can trowel it to keep your arris sharp.
Consider casting in a couple of closed-ended tubes. You can then have stainless dowels projecting from the top of the pillar on which the cap sits to help locate and secure them.