You might want to use a regular chicken crate, but turn it up on it's side (the door will be on the side and it will be about 12 inches wide and 3 feet tall with the long side front and back.) Put thick bedding on the bottom - 4 inches at least. It fits turkeys well, and keeps them from slipping. You can line the crates all up and tie them down so they don't fall over or slip around.
I wouldn't try to make a big pen -- they can slide around too much and hurt a leg or crash into something. Better to have a semi-snug or small pen for each turkey.
Also, keep traveling to a minimum. I wouldn't haul turkeys much more than 2 or 3 hour distance at the most.