  We design and implement 3D lumped element microwave cavities for the coherent
and uniform coupling to electron spins hosted by nitrogen vacancy centers in
diamond. Our design spatially focuses the magnetic field to a small mode
volume. We achieve large homogeneous single spin coupling rates, with an
enhancement of the single spin Rabi frequency of more than one order of
magnitude compared to standard 3D cavities with a fundamental resonance at
\SI{3}{GHz}. Finite element simulations confirm that the magnetic field
component is homogeneous throughout the entire sample volume, with a RMS
deviation of 1.54\%. With a sample containing $10^{17}$ nitrogen vacancy
electron spins we achieve a collective coupling strength of
$\Omega=\SI{12}{MHz}$, a cooperativity factor $C=27$ and clearly enter the
strong coupling regime. This allows to interface a macroscopic spin ensemble
with microwave circuits, and the homogeneous Rabi frequency paves the way to
manipulate the full ensemble population in a coherent way.
