Hello Sara,
Yes, from the image you attached, you have implemented a viable solution for defining Rth,cs. This is not necessary, but should you want to include the thermal capacitance of the case and/or heat sink you have two options. Either you will need to design a hierarchical structure of subsystems to define the thermal layers and use the Ambient Temperature component inside to connect to the nearest external layer (e.g., device sits on Case heat sink block, and via Rth,cs connected to ambient outside of subsystem, which is actually the true heat sink of the system), or directly place a Thermal Capacitor at each node in between.
Regards,
Kris