rush matting
苫 refers to a woven covering made from rushes or straw, typically a mat used for thatching or temporary shelter.
苫 combines the grass radical 艹 with 占, which likely provides the sound. The character originally referred to a thatch or rush mat used for roofing or shelter.
The grass radical 艹 sits above 占, like woven grass or rushes spread over a frame to form a mat or thatch.
For セン, imagine a rush mat spread over a cent (coin) to protect it from the rain: cent -> セン, and the mat covers the coin.