TT #186 · Parshas Shemini · Aliyah 5 · Passuk 32
Chabad.org — Vayikra Chapter 11 with Rashi
English: “Or sack”: a wooden vessel made to remain in one place does not contract impurity in the ordinary manner.
Chagigah 26b.
E-Daf.com — Chagigah 26b (Tsurras Hadaf)
Halachic application — The derashah distinguishes vessels designed for use from those made merely to remain in place.
Stationary wooden vessels are excluded; portability is the comparison to a sack.