Robins like open nesting with a good view of the surroundings. An ideal robin house would have a roof, plexiglass back, a nesting shelf, and a floor slanted slightly forward for drainage. The sides and front open. Coaxing them to leave your porch without closing it off might be difficult. Maybe you could build the bird house near the porch and then enclose the porch with some temporary barrier such as Polypropylene Mesh.