I've built a moped hauler in the back of my truck, which works great. it's made of wood, and has slats in it the size of a moped tire. it has eyelets all over so that i can strap them down in a variety of ways. it's a small truck, but i can haul three of them stand up without them touching. i've taken mopeds all over the country with this setup, and it works great.
basically it's just made of 2x4's stand on their sides and put together slats. i'll try to take a photo to show more detail.