  We present an approach to turbulence closure based on mixing length theory
with three-dimensional fluctuations against a two-dimensional background. This
model is intended to be rapidly computable for implementation in stellar
evolution software and to capture a wide range of relevant phenomena with just
a single free parameter, namely the mixing length. We incorporate magnetic,
rotational, baroclinic and buoyancy effects exactly within the formalism of
linear growth theories with nonlinear decay. We treat differential rotation
effects perturbatively in the corotating frame using a novel controlled
approximation which matches the time evolution of the reference frame to
arbitrary order. We then implement this model in an efficient open source code
and discuss the resulting turbulent stresses and transport coefficients. We
demonstrate that this model exhibits convective, baroclinic and shear
instabilities as well as the magnetorotational instability (MRI). It also
exhibits non-linear saturation behaviour, and we use this to extract the
asymptotic scaling of various transport coefficients in physically interesting
limits.
