I'm having trouble visualizing it the way you're describing, but your A-Frame idea is VERY interesting:
The sides MAKE the roof.
Let's also remember that I have a truck, so worst case scenario, it DOESN'T have to fit into the cabin.
So if we think of an A-frame, one open side butts the TD. If you wanted some decent height, taller than the TD, like you said, you want to go 8 feet. So you would need a small triangular gusset piece that blocks out that open area at the top. Know what I mean? This is where rain gets in.
Now, is there any issue with the open area at the bottom, below the trailer?
Next, the other side of the A can be sealed off with a triangular piece of canvas or vinyl, with snaps.
But here's the biggie:
What happens with the floor? Let's assume we don't care, we just want a place to bunk an extra person and use it for nature calls. But if a big rain comes and it slopes to that area, our feet are soaked. But that's an issue you would have to deal with with any side enclosure, right?
Maybe the coroplast would work for this.