Minecraft wiki entry for Flying_machines, associated text: Dimensions: 8x4 collapsed
Materials: 4 observers, 10 slime blocks, 2 sticky pistons, 2 regular or sticky pistons, 4 note blocks. For seating: 2 fences, 2 minecarts
Usage: To start left or right, click the note block with that arrow (behind you). To stop, spam-click the matching barred note block (ahead of you).
Credits: Sethbling February 29, 2019, "Simple 2-Way Controllable Flying Machine" (Video). YouTube