The world is changing and demanding stronger, lighter, and more versatile materials. Taking advantage of the full potential of these materials also requires versatile manufacturing processes. The in situ forming of a liquid-infused preform (ISFLIP) is a new manufacturing process for fiber-reinforced polymer (FRP) parts with shell shapes. ISFLIP is a hybrid process between vacuum infusion (VI) and diaphragm forming. This paper focuses on the mechanical design and experimental validation of a functional prototype of ISFLIP. The novelty of the design lies especially in a double-diaphragm system that is fundamental to carrying out the forming just after the infusion stage. The double-diaphragm system and other two major subsystems, a vacuum table and an infrared heating grid, were devised to benefit from the operational advantages of ISFLIP. The whole prototype, once constructed, was tested by forming some demonstration components. The result of one of these components, a “C” cross-section FRP profile with two sharp joggles, is finally obtained, proving the feasibility of the prototype.