  A comprehensive study of the frequency dependence of the photon-magnon
coupling for different magnetic samples is made possible with a tuneable
3D--printed re--entrant cavity. Strong coupling is achieved, with values
ranging between 20--140 MHz. The reworked theory, experimentally verified for
the first time here, enables coupling values to be calculated from simulations
alone, enabling future experiments with exotic cavity designs to be precisely
engineered, with no limitations on sample and cavity geometry. Finally, the
requirements of the deep strong coupling regime are shown to be achievable in
such experiments.
