I've seen a swinging divider that had a chain / butt bar on either side. That's what held the divider in place when going down the road. It swings open to load/unload, then chain it in place. I imagine that it would still move a small amount, but not overly much.
Build a removable center bar that slides into a "groove" on the bottom and is held at the top by a locking pin. Without the bar, how are you closing and securing the back doors?
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