The Close-Proximity (CPX) method is a standard methodology (ISO/CD 11819-2) for on-road measurement of tyre/road noise based on the PolyU Mark II Twin-wheeled CPX trailer developed for measurement in Hong Kong urban environment. Its acoustic enclosure is equipped primarily for shielding the contamination from external noise. It would be ideal to be anechoic so as to give a free field radiation condition for generated noise from running tyres; however, this condition is often damaged by multiple internal reflection inside enclosure. As such, a quality acoustic enclosure has to be designed and built to provide minimal background due to multiple reflections. Unfortunately current trial-and-error design practice for modifying enclosure construction for fine tuning of enclosure acoustic field is extremely time consuming and ineffective. This paper reports an alternative approach for designing quality CPX acoustic enclosure using three dimensional finite element method (FEM) simulation and its associated development of modeling of enclosure internal absorption material, source excitation, boundary conditions. The verification and validation of numerical approach by comparing the enclosure acoustics of an existing CPX trailer will be reported. The effort on a numerical parametric study of various conceptual CPX enclosure designs for the search of superior enclosure acoustic quality is then described and discussed.