The Kubelka-Munk model and extensions thereof are extensively used in product development, thanks to their simplicity and the fact that they are invertible. However, their explanative power is limited to distinguishing processes that affect scattering properties from those that affect absorption properties, an example being the frequently observed decrease of the scattering coefficient with calendering. Better understanding of how internal structural changes affect the optical properties requires modelling the composite structure of a paper layer. We propose a modular object-oriented Open Source Monte Carlo simulation platform, Open PaperOpt, which allows different geometrical representations of the inner paper structure. The proposed file formats open for a standardisation that facilitates data exchange between different simulation tools and measuring instruments, and supports a broad dissemination of the simulation software.