The design elements considered during the construction of a thermochemical reactor determine its thermal performance. This current study investigated the effect of design elements, such as boundary layer thickness, insulating materials for the outlet tube design and fluid inlet locations of the frustum, on the thermal performance of a proposed syngas production reactor with incident radiation heat transfer through quartz glass. The P1 radiation approximation model and fluid flow in the shallow path were integrated into a proposed radiation model. The result indicated that inlet mass flow rates from 5 × 10−4 to 14 × 10−4 kg/s increased the temperature in the cavity and the outlet. The fluid inlet located at the top of the quartz glass edges was found to have better thermal performance and maximum average outlet temperature. Insulation for fluid inlets tube above the quartz glass edges of the frustum was very important for the prevention of radiation loss through quartz glass and sedimentation of fluid particles around the quartz glass edge, and the facilitation of fast heat transfer towards the internal part of the reactor. The outlet that was a tube designed using an aluminum oxide-type insulator with a 50 mm boundary layer thickness was found to increase the average outlet temperature of the reactor. This study revealed that fluid entry and exit locations on the frustum and proper fluid outlet design were critical for the thermal performance analysis of the solar thermochemical reactor for heat transfer with quartz glass. Findings from this study will be of relevance to chemical and power engineering sectors, as well as academia.