Medialization thyroplasty is a frequently used surgical treatment for insufficient glottal closure and involves placement of an implant to medialize the vocal fold. Prior studies have been unable to determine optimal implant shape and stiffness. In this study, thyroplasty implants with various medial surface shapes (rectangular, convergent, or divergent) and stiffnesses (Silastic, Gore-Tex, soft silicone of varying stiffness, or hydrogel) were assessed for optimal voice quality in an in vivo canine model of unilateral vocal fold paralysis with graded contralateral neuromuscular stimulation to mimic expected compensation seen in patients with this laryngeal pathology. Across experiments, Silastic rectangular implants consistently result in an improved voice quality metric, indicating high-quality output phonation. These findings have clinical implications for the optimization of thyroplasty implant treatment for speakers with laryngeal pathologies causing glottic insufficiency.