fish trap, weir
筌 centers on a device for trapping fish: a basket-like weir placed in water to catch fish as they swim through.
筌 combines 竹 (bamboo) with 全, which likely contributes the sound. The character refers to a bamboo fish trap, consistent with the bamboo radical.
Bamboo 竹 is woven into a complete 全 enclosure to form a fish trap. Picture a bamboo basket set in a stream, catching fish as they swim inside.
For セン, imagine a thousand (千, せん) fish swimming into the bamboo trap: 千 -> セン, and the trap fills completely.
cylindrical bamboo fishing basket