Overengineered, shmoverengineered! 2x4s are the backbone of any DIY project. They're not putting the trailer into production, they're knocking something together in their backyard.
That brings us back to
Post #42:
Dimensional lumber is universal application. Your budding trailer designer can drop $100 at Lowes Depot, go home, and then figure out what he's going to make as he goes along.