  Helical Majorana edge states at the 2D boundaries of 3D topological
superconductors can be gapped by a surface Zeeman field. Here we study the
effect nested defects imprinted on the Zeeman field can have on the edge
states. We demonstrate that depending on the configuration of the field we can
induce dimensional reduction of gapless Majorana modes from 2D to 1D or
quasi-0D at magnetic domain walls. We determine the nature of the Majorana
localisation on these defects as a function of the magnitude and configuration
of the Zeeman field. Finally, we observe a generalisation of the index theorem
governing the number of gapless modes at the interface between topologically
non-trivial systems with partial Chern numbers.
