Let me highly recommend you read or join the form from Mikechell.com . They have endless info that will be helpful for your project. Most of their members build really great Teardrop trailers. You can join for FREE. I am just a newbie over there, so I will let you know more details when I figure it all out. But if you click on the link below, you can get help with your cargo trailer project. Some of the projects are 20+ pages long.
The only bad thing about reading over there, is that I get more crazy ideas and spend less time building my project, what the hell, its a lot of fun seeing other projects.
Building cargo trailers