I am the Director of Sales and Marketing for Tallysman. We recommend a ground plane of 10cm. The ground plane should be symmetrical about the antenna and perfectly flat. The cable exiting the antenna should be routed under the ground plane as close to the housing of the antenna as possible.
It doesn’t matter if the ground plane is made from aluminum, copper, or any other metal. The important feature is that the material is an RF reflective material.
Thickness also doesn’t matter.