The Uruguayan Government this week completed plans for the biggest naval air base south of the Panama Canal. To be built on Laguna Negra, a lake close to the Atlantic Coast, 160 miles northeast of Montevideo, it will command the broad mouth of the Rio de la Plata, a wide sweep of the South Atlantic. It will be constructed with U.S. funds lent and given to Uruguay for national defense. Like other Uruguayan ports, Laguna Negra Base will be open to U.S.. forces in case of war with the Axis