After all stanchions have been created, move them forward the same 3 inches. Mirror the stanchions across centerline before lofting to ensure the lofted surface comes tangent at centerline. Loft a surface through the aft stanchions. With the surfaces lofted through the side and aft stanchions, trim each to the other and join them into a polysurface. To make the corner, use the FilletSrf command and specify a radius of 3 feet.