Dealing with small sidewall and lip on trailer

Hi there,
I have been given a trailer that I’m going to use for a pseudo-standy-squareback.
The trailer has a metal edge down the long edges - see photo .
I don’t want to cut off the whole thing because it seems fairly structural.
Originally I planned to build the wall to encase it - see sketch - but this will make the walls slightly too wide apart to use a sheet of 5’ BB on the roof. The width across the trailer to the outside of the vertical part of the metal side is 5’ exactly.
Alternatively I could build the walls inside the metal side, cut off the lip, and add a flashing on top of the cut edge and under the metal skin.
Another issue is that I want to extend the length 1’ past the end of the trailer to accommodate some of my wife’s wish list: galley, full bed foldable along length, bunk across front for child. If the metal side is exposed it won’t cover the extra length but maybe I can be creative with some checker plate or similar.
I’d welcome your thoughts on these or other options.
Thank you,
James
I have been given a trailer that I’m going to use for a pseudo-standy-squareback.
The trailer has a metal edge down the long edges - see photo .
I don’t want to cut off the whole thing because it seems fairly structural.
Originally I planned to build the wall to encase it - see sketch - but this will make the walls slightly too wide apart to use a sheet of 5’ BB on the roof. The width across the trailer to the outside of the vertical part of the metal side is 5’ exactly.
Alternatively I could build the walls inside the metal side, cut off the lip, and add a flashing on top of the cut edge and under the metal skin.
Another issue is that I want to extend the length 1’ past the end of the trailer to accommodate some of my wife’s wish list: galley, full bed foldable along length, bunk across front for child. If the metal side is exposed it won’t cover the extra length but maybe I can be creative with some checker plate or similar.
I’d welcome your thoughts on these or other options.
Thank you,
James