I had a suggestion elsewhere of using a box design where the box itself was the "frame" or "structure" in and of itself.
I think if the box was formed correctly, or if the box was assembled well (bolted, riveted, or welded to lock everything together well) that could very well work. However, the stress points and more structural areas - like the hitch draw bar connection and spots for the wheel/axle attachments - would need beefed up/reinforced in a way to distribute the forces to the "box".
That sounds like it would work. However, if I do that it will restrict the versatility of the trailer. If I build a frame that can stand alone/work by itself without the box then I can stretch that and have a more utilitarian flatbed style trailer. Whether or not I have sides or guard rails is a moot point in the idea phase here. I am just thinking through the possibilities and I like the idea of the stand-alone frame with the box on top so I can split them and use the frame without the box.