theres quite a few on youtube https://www.youtube.com/watch?v=IVLc_1PE6Qk what I notice is a lot of aluminum to keep weight down.
Here's the flatbed I built a couple years ago. I still haven't built a box for it, let alone a dump box, but it's been a great trailer for me. http://www.arboristsite.com/community/threads/woods-trailer-2-0.247169/
Good looking build! Thanks for sharing!
I have one of these: ATV Trailers Heavy Duty Tandem Axle Model 7550ATV by Country ATV.
The price seemed pretty reasonable to me. If nothing else, you might get some ideas for building your own.