In this study a shielding effectiveness (SE) measurement procedure is presented, utilizing a small footprint software-defined radio (SDR) device that enables the realization of a spectrum analysis of medium-sized cavities in-situ. It additionally makes use of a standalone reference radiator, enabling an inside-out measurement procedure to be executed. Because of this, no physical link exists between the inside of the enclosure and the outside, guarantying a reduction of possible coupling paths and removing the necessity to physically alter the enclosures original design. This technique was experimentally validated using a standard commercial-of-the-shelf (COTS) 19-inch cabinet, in which two antenna configurations were considered, in both position and polarization, enabling a performance assessment when in-situ restrictions need to be considered.