'Planted' pictures still to come - Ken dropped off the cap pieces and I installed it this weekend. It was a bit tedious because I was going for a centered, snug fit and that meant that it wasn't as simple as finding center and slapping down the MFS.
After cutting the mortice, I fixed the flat piece with four stainless screws driven down into the 2x4 uprights. Then I drove four screws upward into the pyramid section from below. The two faces didn't mate perfectly, possibly because screwing down the flat piece may have distorted it a bit. Since I had already glued a number of other joints, I broke out the TiteBond II again. Put the clamps to it and its looking pretty good now. I just need to sand the edges to flush them and remove some glue residue.
In terms of performance, the plants seem mostly to like the planter. I am still getting flowers and strawberries. The top tier soil does tend to dry out more quickly because it has more exposure to air and sunlight and is also just a smaller mass of soil. Those plants are 'ok', but not as happy as tenants on the lower floors. I'm hoping that if I can get the top plants to bulk up in size, the foliage will shade the soil. Soil washout was a bit of a concern to me, but so far it is seeming not to be much of an issue. It took about half a yard of soil to fill.
Thanks again to Ken for the help on this! The top really finishes it nicely. Everyone who has seen it really likes it.