VPN, I am not Tom but I have some suggestions. First, while placement is important, I don't think there are any one-size-fits-all generalizations to be made. In my case, with a Duelund CAST crossover cap, I started out with 3 brass cones under its flat surface. This combination added some life and speed which was necessary at first because the cap starts out rather dark and lifeless. As it broke in, I tried other arrangements and for a while liked having just 1 cone close to one edge of the cap leaving the other side of the cap touching the mounting surface at just one point. That worked fine for a while but eventually I went to 3 small cubes of Baltic Birch plywood each one about 3/4" on a side. That gave to my ears the most natural balance. So you need to try different supports but be prepared to reach different conclusions as to what sounds best as the CAST cap breaks in.
For my big North Creek inductors, I used the same 3 cube supports but the mounting arrangement did not make as much difference.
For my big North Creek inductors, I used the same 3 cube supports but the mounting arrangement did not make as much difference.