This work investigates numerically a natural convection flow in a rectangular differentially heated cavity. The aspect ratio of the cavity (height over width) is 4, the working fluid is air and the Rayleigh number based on the height of the cavity is 9×107. A large rectangular obstacle is placed in the center of the cavity. The influence of the size of that obstacle and the effect of a new geometry (obtained by extrusion of two rectangular forms from the obstacle) on the flow dynamics and heat transfers are investigated. The global and local heat transfers modifications, compared to the case without obstacle, are highlighted for several configurations. In such a flow regime, an optimal geometry and a solid-to-fluid conductivity ratio are determined. This geometry leads, for an adiabatic obstacle, to a global heat transfer increase of 4.39% with a local heat transfer increase up to 80%.