A manifold structure and method of forming a structure having at least one enclosed cavity includes using an ultrasonic additive manufacturing (UAM) process to build up a solid component, forming a cavity in the solid component, filling the cavity with a sacrificial material, using a UAM process to build up a finstock layer over the cavity filled with the powder material to enclose the cavity and form the enclosed cavity, and removing the sacrificial material from the enclosed cavity after the finstock layer is ultrasonically welded to the solid component. The sacrificial material has an adequate density to support the UAM process of forming the finstock layer over the cavity and the material may be removed from the enclosed cavity, resulting in an enclosed cavity having smooth surfaces with an optimal fluid flow area therethrough.