The Tenerife Microwave Spectrometer (TMS) is a ground-based radio-spectrometer that will take absolute measurements of the sky between 10–20 GHz. To ensure the sensitivity and immunity to systematic errors of these measurements, TMS includes an internal calibration system optimised for the TMS band, and cooled down to 4 K. It consists of an Aluminium core, composed of a baseplate and a bed of pyramidal elements coated with an absorber material and a metallic shield. The absorber coating is made of a commercial resin ECCOSORB CR/MF 117. To achieve the high stability (± 1 mK/h), temperature homogeneity (thermal gradients ΔT ≤ 25 mK), and emissivity (e ≥ 0.999) requirements of the reference unit, careful consideration has been given to the RF and thermal properties of the materials, as well as their geometry. In summary, this paper presents a comprehensive account of the design, characterisation, and test results of the TMS reference system.