Custom Keychains & Keylights, Hamburger Key Chain - Stress Reliever - Polyurethane, Hamburger shape stress reliever key chain, 1 1/2" diameter x 1 1/8". Due to their handcrafted nature, stress ball sizes, colors and textures may vary. Factory cannot guarantee consistent imprints or PMS matches. Material: Polyurethane. ... #LKC-HB07 $1.87/each.