An endovascular prosthesis includes a tubular body and a mobile external coupling. The tubular body includes a graft material and stents coupled thereto, and forms a lumen therethrough. The mobile external coupling includes a graft material, extends outwardly from the tubular body, and is generally frustoconically shaped. The mobile external coupling includes a base coupled to the tubular body, a top spaced from the tubular body, and a coupling lumen disposed between the base and the top that is in flow communication with the body lumen. An annular support wireform is coupled to the mobile external coupling, and is formed into a sinusoidal configuration having a plurality of opposing first crowns and second crowns, the first crowns of the support wireform extending around of the top of the mobile external coupling. The coupling graft material extending between the second crowns of the support wireform and the tubular body is unsupported.