A forming jig includes a grid pattern of support walls arranged on a support base and having upper free ends located along an imaginary curved surface, and modular sectional profile members arranged on the walls to enclose a vacuum chamber within the jig. Grooves, channels and air passages between neighboring profile members communicate into the vacuum chamber. The outer surface of the profile members matches the intended inner surface of a structural component to be fabricated using the jig. In the fabrication process, a film is applied to the outer surface to pre-establish a vacuum, the film is removed while simultaneously a vacuum skin is applied, stringer members are set into the grooves, fiber skin layers are laid up, a sealant is applied around the perimeter, a structural shell is vacuumed onto the skin layers, and then this preformed component is removed from the jig, injected with resin unless the skin layers were pre-impregnated with resin, and cured.