brick
塼 centers on a brick: a block of fired clay used in building.
塼 combines 土 (earth) with 專, which likely contributes the sound and relates to shaping or concentrating clay into a brick.
The left side is earth 土, and the right side 專 suggests something concentrated or specialized. Picture earth being specially shaped and fired into a brick.
For セン, imagine a brick being sent through a kiln on a conveyor belt: sent -> セン, and the brick emerges solid and ready to build with.