We present a simplified coupled mode theory (CMT), suited for high losses, to describe ultra-broadband THz generation through optical rectification (OR) of fs infrared pulses in waveguides. We derive a new expression that incorporates loss effects into the coherence length for OR. The simplified approach reproduces the results of a computationally rigorous integral CMT that must be used for broadband THz generation. With the new model we perform a parametric study to establish the optimal conditions for OR in symmetric, five-layer, metal/cladding/core structures with electro optic polymer cores. We find conversion efficiencies as high as 35 × 10⁻⁴ W⁻¹ and bandwidths up to 20 THz when pumping at 1900 nm. We find that low-loss-cladding layers enhance the efficiency for phase-matched structures, increase the interaction length, and improve the stability of the efficiency with respect to variations in waveguide parameters.