Greg's hardside idea shows up the problem that I had to work around with my full-length-roof version of the Winter Warrior - you have to pay attention to the arcs around the hinge to get the lengths right.
The top diagram shows what happens to Greg's design when lowered - the bottom half of the (red/blue) lid tries to drive itself through the (green) base. The bottom diagram shows how much it needs to overhang the base if it's going to be hinged down.
And what's with this aluminum-faced coroplast? If you have to pedal it around, the aluminum is going to be a lot of added weight. What's wrong with straight coroplast? The lower bodywork on my motorbike was coroplast and duct tape for a long time and it did fine in heavy rain at 90mph.
So for example, the trailer top in corroplast might need a 1" deep coroplast rib running across it every 12-16", held in place with duct tape, to stop it deflecting. That's a little over a square foot of extra coroplast or 1/3 of a pound (plus that duct tape!). Two 0.040 skins of aluminum weight 1.1 pounds/sqft or about 18 pounds for the whole top - even the 0.016 skins weigh 7 pounds for just the top.
But then again, youth will generally overcome adversity, so a heavy trailer in the hand is better than an unfinished light trailer!
Andrew