Build the tee-slotted bar
The three views below show a bar with a T-shaped slot running through it: a narrow opening at the top widening into a channel below. Build it from the full block. Two cuts make this shape, and the wider one removes material the narrow one alone would have left.
The part, in three views
corner 1 of 4
Click a face to cut that block away. Right-click to put a block back. Use Turn left/right to walk around the part: a feature on the far side cannot be reached from here.