Applying this principle where the open space is one through which goats could pass: Shabbat 101a-b
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
From the top of a pole, if it thins as it descends: Shabbat 101a