We present a new waveguide thermal isolator, based on a simple gap in a standard rectangular waveguide, developed for the $Q$ -band cryogenic receiver of the Sardinia Radio Telescope. The proposed component is very compact and consists of a single module, including both the horizontal and vertical polarization channels and the noise injection channel. The electromagnetic and thermal characteristics of this isolator have been fully investigated. High return loss and low insertion loss are obtained solely by a proper selection of the waveguide wall thickness, thus resulting in a very simple design, easily scalable in the whole microwave range. High robustness to misalignment and excellent thermal performance are achieved. A prototype of the proposed isolator has been manufactured and tested at room temperature, showing a very good agreement with predicted performance.