To achieve substantial emission reductions, widespread low-carbon technology adoption is vital. The process by which new technologies are developed and adopted and how their costs evolve is critical to understanding decarbonization. Modeling of this process requires a tool that realistically describes several phenomena related to technology adoption in different sectors, including technology diffusion, investment decisions, evolution of technology costs, and technology lock-in, among others. This paper introduces FTT-FLEX, a simplification of the Future Technology Transformation model. FTT-FLEX is suitable for application as a single-country, standalone tool or in connection with a macroeconomic model. FTT-FLEX captures the core country-level of features of Future Technology Transformation (knowledge spillovers as the driver of technology cost and inertia in the adoption of new technologies) as they pertain to an individual country and greatly reduces the data required as compared with the global Future Technology Transformation model. As presented, FTT-FLEX is a natural complement to country-specific macroeconomic models that analyze the decarbonization of key emitting sectors in small developing countries. The utility of FTT-FLEX is demonstrated by a decarbonization analysis for the power sector in Guinea-Bissau.