Nice start and well thought out . That being said you should learn to TIG weld and build it out of alluminium tube 6061 with about a 3 /32 " wall. Stay withe bottom design make it like the trailer swing arm, I would make a tarp top out of your choice of water proof material and strap it down to the lower portion of the trailer. the front end of the trailer will need bridging support for those hime joints. That vertical tubing wouldn't last more than a thousand miles if that the constant flexing with a full load (200 lbs?) will snap that right off. Hope this helps stay safe!
