The Configurable Slit Unit (CSU) for EMIR shall enable the possibility to generate a multi-slit configuration, a long slit, or an imaging aperture at the entrance focal plane of the GTC-EMIR instrument. The CSU is therefore a cryogenic reconfigurable slit mechanism. It contains 110 sliding bars which can be positioned within the 340x340mm wide aperture of EMIR's instrument field of view. Based on the results which haven been obtained in the previously performed demonstration programme, the current developments have focused on the optimization of the performance and up scaling of the construction. Major progress and improvement have been made on the position measurement of the slits, as a result of conceptual improvements in the measurement system but also as a result of fundamental hardware changes. Furthermore an improvement of the thermal household of the instrument will contribute to the position stability performance. Main development advances on several aspects of the instrument design are presented. This work is performed under contract of the Instituto de Astrofisica de Canarias as part of the development of the EMIR instrument to be installed at the GTC telescope.