If they are mostly road bikes, and the handlebar heights and seat heights are roughly the same, then do this: find two long pipes or sticks. Lay one across all the handlebars and zip-tie the tops of the handlebars to the stick tight. This will keep all the bikes upright and the forks pointing straight ahead. Now lay the other stick across all the saddles. Use bungie cords to strap it down to each of the saddle. This will keep the backs of the bikes from moving around. If you can anchor the two sticks to the walls of the moving van, or cut the sticks to the exact width of the inside of the van, even better.