The truck is long gone; and at the Country Fair this year I learned that the cabin/camper which was up on blocks for years in the Coast Range above Ona Beach had a tree fall on it.
It was in the book Shelter (on page 92, I hear).
The curve you're showing with flat sides would induce vortexes. The top and sides need to balance. It's actually easier for me to take screenshots in a 3D application than overpaint things in Photoshop. Easier yet to recycle old stuff.
Here're your choices:
With that long bed and short overhang, you may need to go to something like this:
