Accurately determining the early Post-Mortem Interval (PMI), the period shortly after death, is critical for reconstructing the timeline of suspected crimes. The PHOEBE model, a new thermodynamic finite-difference model that simulates body cooling, significantly improves PMI estimations but is highly sensitive to the thermal conductivity of the deceased’s clothing. This study introduces the Therminus-K3 prototype, an affordable and portable Guarded Hot Plate apparatus, designed specifically to support the PHOEBE model by providing rapid, non- destructive thermal conductivity measurements of clothing garments. The Therminus-K3 features a compact Hot Stage with resistive PCB heaters, a Cold Stage with thermoelectric coolers, and a Compression Stage that replicates real-world fabric compression conditions. This design allows for rapid attainment of steady-state conditions and thermal conductivity measurements within 25 minutes, essential for time-sensitive forensic investigations. Preliminary results suggest that the Therminus-K3 prototype measures with a precision of less than ±1% and a measurement uncertainty below ±1.5%, but further testing is required to confirm its overall accuracy. With a material cost of just €404.39, the Therminus-K3 offers forensic practitioners a more affordable option to accurately measure the thermal conductivity of clothing, thereby enhancing the accuracy of PMI estimations. Initial tests suggest that integrating the Therminus-K3 with the PHOEBE model could improve PMI estimations by over 30 minutes. Its ongoing development holds significant potential to further impact and improve the determination of time of death in forensic scenarios in a more efficient and reliable manner.