Our research focuses on leveraging developments in computing hardware and modern particle physics detectors to produce a simple, effective, robust, modular threat assessment system to non-destructively detect concealed contraband radioactive materials. Mobility and weather resistance are central to our design. The baseline design calls for the use of single board computers (SBC) and charged particle detectors that are very mature and reliable in their design and operation. Our particle probe for screening materials is naturally-occurring cosmic-ray muons. We aimed to develop this Lego-like system to demonstrate simple tomography of brick-size shielding for transport of radioactive materials within packaging as large as a 55-gallon oil drum. The goal was to significantly reduce costs compared to custom detector system designs, by using commercial off-the-shelf components. System design focused on producing equipment that a trained but non-expert operator can operate.