Abstract: A double-walled container formed of synthetic resinous material, including generally hemispherically-shaped outer and inner walls nested together with a gap between the walls, a flange extending from the outer periphery of the inner wall to the top edge of the outer wall, the flange and outer wall being permanently joined together so as to form the two walls into a unitary structure. Preferably, the walls are joined together by ultrasonic welding. The top edge of the outer wall is formed with a stepped configuration defining a seat which accommodates a sealing portion of the flange, the sealing portion being fused to the seat. The sealing portion of the flange may either be continuous or formed of individual projections spaced apart along the circumference of the flange. The top surface of the flange may either be flush with, or in a plane above, the uppermost surface of the top edge of the outer wall.