Minecraft wiki entry for Flying_machines, associated text: A flying machine can be designed to not only carry a player along, but allow them to control the machine in flight, starting, stopping, or changing directions. Often due to lag, slime engines can bug allowing the player to fall through the machine. This can be solved by placing the player in a vehicle such as a minecart, by standing on top of a honey block, or by standing inside a Composter.
Note: Placing a minecart in any such build will require a temporary rail, perhaps atop a temporary block for convenience.