The strangers is already plural so the possessive form is strangers' adding an apostrophe at the end. However odd it may look it is correct and you do not add another s or 'es to the end.
The plural possessive of "stranger" is "strangers'".
Strangers' hats.
The word "strangers'" is plural possessive, indicating something that belongs to multiple strangers.
The plural is relatives. The plural possessive is relatives'.
The plural is lads. The plural possessive is lads'.
The plural form is Mexicans. The plural possessive is Mexicans'.
Strangers' hats.
Coaster's is singular possessive. The plural is coasters, the plural possessive is coasters'.
The correct form is "its" for the possessive form in the plural. "Its" is used for both the singular and plural possessive forms, without an apostrophe.
The plural is briefs. The plural possessive is briefs'.
The plural is lads. The plural possessive is lads'.
The plural is trollies. The plural possessive is trollies'.
The plural is stations. The plural possessive is stations'.
The plural is relatives. The plural possessive is relatives'.
Museum is singular. Museum's is singular possessive. Museums is plural. Museums' is plural possessive.
The singular possessive form is heart's; the plural possessive form is hearts'.
The plural form is homes; the plural possessive is homes'.
The plural of rose is roses. The plural possessive is roses'.