Preventing downtube cable stops from sliding out of place
That calls for a shim. Cut a narrow strip from a drinks can, about 3-5cm long and lay it under the cable stop before re-tightening the screw. Make sure all the surfaces are clean and degreased. The soft aluminium of the shim will 'absorb' the remaining roughness of the tubes and the cable-stop and make slipping more difficult. (I've solved the problem of the brake lever rotating around a steel handlebar in this fashion.)