gate bar
閂 centers on a horizontal bar used to secure a gate or door, preventing it from being opened.
閂 combines 門 (gate) with 一 (one), likely representing a single bar placed across a gate to lock it. The exact historical development is uncertain, but the visual logic is clear.
Inside the gate 門, a single horizontal bar 一 is placed across to lock it. Picture a sturdy wooden bar dropped into brackets on a gate.
For サン, imagine the sun (サン) shining on the gate bar as you slide it into place: sun -> サン.