We developed an efficient hybrid mode expansion method to study the maximum tunneling current as a function of the external magnetic field for a 2D large area lateral window junction. We consider the inhomogeneity in the critical current density, which is taken a piecewise constant. The natural modes of the expansion in y, are the linearized eigen-modes around a static solution which satisfies the 1D sine-Gordon equation with the critical current variation in y, and the boundary conditions determined by the overlap component of the bias current, which can be inline or overlap like. The magnetic field along with the inline component of the bias current enters as a boundary condition on the modal amplitudes. We obtain fast convergent results and for a ratio of idle to window widths of w0/w=4 (in units of λJ), only two modes are needed. A simple scaling is obtained for the maximum tunneling current as we vary the idle region width. We also present the linear electromagnetic waveguide modes taking into account the variation normal to the waveguide of the critical current and the capacitance.