  Recent interest in primordial black holes as a possible dark matter candidate
has motivated the reanalysis of previous methods for constraining massive
astrophysical compact objects in the Milky Way halo and beyond. In order to
derive these constraints, a model for the dark matter distribution around the
Milky Way must be used. Previous microlensing searches have assumed a
semi-isothermal density sphere for this task. We show this model is no longer
consistent with data from the Milky Way rotation curve, and test two
replacement models, namely NFW and power-law. The power-law model is the most
flexible as it can break spherical symmetry, and best fits the data. Thus, we
recommend the power-law model as a replacement, although it still lacks the
flexibility to fully encapsulate all possible shapes of the Milky Way halo. We
then use the power-law model to rederive some previous microlensing constraints
in the literature, while propagating the primary halo-shape uncertainties
through to our final constraints. Our analysis reveals that the microlensing
constraints towards the Large Magellanic Cloud weaken somewhat for MACHO masses
around $10\, M_\odot$ when this uncertainty is taken into account, but the
constraints tighten at lower masses. Exploring some of the simplifying
assumptions of previous constraints we also study the effect of wide mass
distributions of compact halo objects, as well as the effect of spatial
clustering on microlensing constraints. We find that both effects induce a
shift in the constraints towards smaller masses, and can effectively remove the
microlensing constraints from $M \sim 1-10 M_\odot$ for certain MACHO
populations.
