Dade County, Fla., seeking a way to shelter its massive influx of Nicaraguans, unveiled a proposal that would put up the homeless refugees in trailers on 3.5 acres left over on a prison site. The plan also calls for housing the refugees and other area homeless at a Miami facility under renovation, and could provide hundreds of people with temporary shelter, said County Manager Joaquin Avino, who announced the proposal. Avino said he hopes to obtain 120 trailers from the federal government and place them on property left over after construction of the West Dade Correctional Center. Immigration and Naturalization Service officials estimate about 300 Nicaraguans a week are fleeing the war-torn country and entering South Florida and that 100,000 are expected to reach Dade County in the next 18 months. County officials called an emergency meeting of their Immigration Advisory Committee for Wednesday to discuss the problem. .