Sunspot penumbrae harbor highly structured magnetic fields and flows. The moving flux tube model offers an explanation for several observed phenomena, e.g. the Evershed effect and bright penumbral grains. In this work, we present a generalized 3D model that embeds an arbitrarily shaped flux tube in a stratified magnetized atmosphere. The new model is a versatile tool to calculate the spectral signature of flux tubes in the penumbra and especially make predictions about the flow speed and tube inclination from observed maps of the net circular polarization (NCP). As a first result, we find that the inclination of downflows in the outer penumbra must be shallower than approx. 15°.