Build the slotted bar
The three views below show a bar with a slot cut straight across it, from front face to back face. Build it from the full block. Because the slot runs the whole depth, the top view shows it as a plain opening rather than as hidden detail.
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.