I know that for sentences such as "this is a port that has been closed for 20 years" I could use 的 or start another phrase. Is it this way for "which" as well?
Yes, it is. In Chinese, dependent sentences are not so common, and most phrases could be used as adj. with '的'.
Otherwise, you could also start another sentence to describe the meaning in 'which' clause, which could be separated by comma.