This paper presents the results of the ESA funded C-MAC activity, started mid-2015 and currently in the assembly and testing phase. The objective of the activity is to develop an innovative and low-cost ceramic-based physics package (PP) for ultra-low power miniature atomic clocks (MACs) with flat form factor for future portable devices (GNSS receiver, smartphone, tablet, laptop, etc.) and high-end demanding applications like onboard satellites.