Accurate pre-launch calibration of the MetOp-SG (Meteorological-Operations Second-Generation) Microwave Sounder (MWS) remote sounding instrument is critical in achieving an operational radiometric temperature uncertainty of 0.1 K. The RAL Space department at the STFC Rutherford Appleton Laboratory is working with Airbus Defence and Space (UK) to a develop calibration system for MWS. This system sets a new level of required calibration accuracy for (sub-) millimetre wave space-borne instruments, and needs blackbody targets with a temperature stability of 0.05 K over 5 minutes and 0.5 K over 1 hour for the temperature range 80 K to 335 K, with a return loss requirement of 45 dB. Achieving the required target diameters of 0.5 m presents significant design and manufacturing challenges.