Applying this principle where the open space is one through which fish could pass: Shabbat 101a-b
From the slanted walls of a boat: Shabbat 101a
From a basket atop a pole: Shabbat 101a; Eruvin 33b
From the top of a pole, if it thins as it descends: Shabbat 101a
Making a hole in a broad beam which overhangs water, and considering the sides of the beam as walls descending to the water, to permit a person to draw water through that hole: Eruvin 86b-87a
Considering the walls of a house as though they extend upward as walls around the roof: Eruvin 89a-b