I hear you on the cargo trailer challenges. I doubled up the roof rafters on mine because the annual snow loads were causing it to sag. It looks like your roof is flat? Mine is, and at one point I was considering trading for a different trailer with a bit of a dome shape in the roof, thinking it would shed better plus have a stronger structure with more of an arch. But instead I just sistered flat plate bars to the rafters. That seems to be holding up well.
And I struggle with locks getting frozen as well. I like the flap idea and will try that.