A modular fabrication tool for prefabricating insulation for a duct segment includes at least three interchangeable and detachably connected panels, each panel of the at least three panels corresponding to a respective side of the duct segment. Each panel includes at least one of a connecting bar extending beyond a periphery of the respective panel or a connecting track. A connecting bar of one panel is received by a connecting track of a neighboring panel when two neighboring panels are properly positioned, adjoined together, whereby a retaining pin of the tool is advanceable through aligned, respective apertures of the connecting track and the connecting bar received therein.