Motorcycles require careful consideration when loading. The problem with motorcycles lies not just in protecting them from damage, but in keeping them secured so they don't damage other items. The average street bike weighs hundreds of pounds and can be a serious threat in turns and hard stops made by the driver of the truck.
It's possible to construct a compartment for motorcycles by using one or more sturdy pieces of wood to span the distance between items on either side of the motorcycle.
Consider placing either the front or back wheel directly against the front wall to absorb force in the event of a sudden application of the brakes.
The motorcycle should be prevented from moving by placing soft, non-abrasive items tightly between the motorcycle and the objects on either side. Immobilize the steering to further decrease movement.
Ensure you omply with requirements for draining fuel, oil, and disconnecting the battery. Check with your rental company for details.