The "long and narrow oblong box" that Bill and Henry tow across the Klondike is the coffin that carries the remains of Lord Alfred, an aristocrat who died on the trail. His coffin symbolizes death, highlighting the fragility of life in the Northland.
The Oblong Box/ Coffin Symbol Timeline in White Fang
The timeline below shows where the symbol The Oblong Box/ Coffin appears in White Fang. The colored dots and icons indicate which themes are associated with that appearance.
Part 1, Chapter 1