Monolithic scintillator-based PET detector modules offer depth of interaction capability and lower price relative to conventional pixelated crystal detectors. However, making such constructions poses several problems that affect light extraction efficiency and accuracy of position determination. In this work we examine potential detector geometries proposed to solve these problems. The geometries were tested by our novel measurement method that utilizes point-like excitation of the scintillator. With the applied methods we successfully increased the light extraction with a factor of two relative to a conventional monolithic PET detector. The measurements were compared to results of complete optical CAD models of the investigated arrangements. We concluded that our optical CAD tool can be used to further optimize the current constructions.