Mold systems for molding fluid-filled elements include a lower mold body having a lower mold surface and an upper mold body having an upper mold surface. A mold apparatus is disposed between the lower and upper mold bodies. This mold apparatus includes: (i) a first arm providing a first mold surface extending between first and second ends of the first arm, and (ii) a second arm providing a second mold surface extending between first and second ends of the second arm. The first and second mold surfaces have surface contours configured to form molded undercut surface features of the fluid-filled element. Methods of molding fluid-filled elements using such mold systems also are described.