We report the design of an experiment that aims to constrain, over a-few-year timescale, the fractional temporal variation of the proton-to-electron mass ratio, β=mp/me , at a level of 10-15/yr by means of a spectroscopic frequency measurement on a beam of cold CF3H molecules. This is extracted from a buffer-gas-cooling source and then collimated by means of an electrostatic hexapole lens. Employed in a two-photon Ramsey-fringes interrogation scheme, the probe source is based on a mid-infrared quantum cascade laser, phase-locked to a specially-developed optical frequency comb that is ultimately referenced to the Cs primary standard via an optical fiber link.