Antenna ground plate metal type

Hello, I would like to know what type of metal and diameter should I use for my antenna ground plate to get the best result.

Also, can it be a square shape?

Square vill work too, I use 40 by 40 cm iron, because I have magnetic mount antenna.
Most metal types will go.
Ublox deliver a round plate diameter 16 cm as far as I remember.
Purpose of plate is to avoid signal reflection from underside/ground , that could give same GPS signal, but with two different receiving times, and impossible to calculate position and rtk from it.

Think in the f9p literature it says a ground plane of 100mm