This paper deals with multilayer and fine thermal control of an optical reference cavity for space applications. The cavity, made of ultralow-expansion glass, must be kept close to the zero-expansion temperature (ZET) of the glass (near room temperature). The target can only be met by active control, while leaving the cavity free of sensors and actuators. This is achieved by applying two concepts: thermal bath and reference thermal sink, the latter allowing the ZET to be reached by heaters in a wide range of the environment. Guidelines for cavity design and thermal control implementation are detailed together with preliminary experimental results.