A sonotrode and anvil are adapted for ultrasonic welding of work pieces, to produce a narrower weld region that exhibits greater durability, permitting use of less material per package. The horn-to-anvil contact is through a plurality of energy directors arranged into a three-dimensional grid pattern to be capable of distributed vibration-transmissive contact. The energy directors include a series of plateau surfaces regularly spaced apart in a first direction, and in a second direction that is orthogonal to the first direction, to form the grid pattern. The energy directors of the horn are configured to interlock with the energy directors of the anvil. The rectangular-shaped plateaus are spaced apart by angled side-surfaces that form valleys. A stepped transition to a corresponding region of reduced height for the energy directors of the sonotrode and anvil may form a cosmetic seal region with a lesser integrity, in addition to the main barrier seal.