  We present a few--mode Galerkin model for convection in binary fluid layers
subject to an approximation to realistic horizontal boundary conditions at
positive separation ratios. The model exhibits convection patterns in form of
rolls and squares. The stable squares at onset develop into stable rolls at
higher thermal driving. In between, a regime of a so-called crossroll structure
is found. The results of our few--mode model are in good agreement with both
experiments and numerical multi--mode simulations.
