The reciprocity principle is widely used for acoustic measurements, including three-dimensionally printed waveguide transfer functions. Previous studies revealed unexpected deviations in the 1-4 kHz range, likely due to secondary transmission paths. This study tests the effectiveness of a sand layer in reducing these paths and evaluates measurement variability and robustness to harmonic distortion. The method, tested on an affected replica and two additional replicas, shows that the sand layer effectively minimizes secondary transmission paths. The measurement system proves robust against harmonic distortion, ensuring good reproducibility, with results compared to simulations for validation.