I like your well thought out approach, I'll be watching.
Google results ...
The heart of the trailer industry is in the Elkhart area. I hope it's close to you. I think there would be some professionals eager to make the best of your design for you, at lower cargo trailer prices. Then finish yourself or use custom guy.
If it were my chips that are all in, I would have the shell professionally and custom built. For peace of mind on the road.