You can make these "boxes" thingies you speak about out of wood. Or even metal. Or maybe something else.
Seriously, though, I'm just messin' with ya. Welcome to ExPo, Gene!
Without a pic of the trailer, it's hard to make a decent recommendation. I think something like the attached diagram can be mounted on the tounge of most trailers. You could build it and increase its size depending on your needs and the space you have avail.
Beyond that, is a simple hinged-lid box mounted in the trailer. Quick and easy build, man.
How about show a pic of the trailer and the motor you're looking to carry. Might feed our brains a little so we can help more.
Thx!