I have an old Hot Wheels styled box trailer that fits the figure near perfectly. I'd suggest looking through the cheaper car toys to find one since most of the more modern ones are rather stylized.
Sorry, camera's charging. I'll try to shoot a pic in the AM. The trailer isn't wide enough for another car.
I have the Hot Wheels Truckin' Transporters trailer used for the Cyb. Armorhide custom trailer (that Reprolabels made a special freebie set for a few years back). It looks like it might be about the right size for Cyberverse Prime... Click to embiggen any of those images... As you can see in the last picture, the trailer's kingpin sits too far back to fit into Prime's 5th wheel, but you could move it forward (where the screw is on the underside of the trailer)... If you'd like, hit me up with a PM; I think I've still got another untouched trailer around here somewhere...
i like it im making a photobucket account right now so i can show you what i found it fits smooth and level