A disclosed seismic source assembly includes a body having a cavity and a seismic source positioned in the cavity. The cavity is in fluid communication with the water via an aperture oriented in a first direction. One or more surfaces of the body define a water contact significantly larger than an area of the aperture and on a side opposite the first direction. A described method includes forming a source assembly by: providing a cavity having an aperture for transmitting seismic waves; rigidly attaching a base to a side of the cavity opposite the aperture, where a transverse area of the base is significantly larger than an area of the aperture; and positioning a seismic source in the cavity. The source assembly is submerged in the water and triggered.