Slow, looking for something else, I came across this, which is almost exactly what you want. By taking a piece of the sidewall up with the hatch, the joint becomes a straight line that the sidewalls can be hinged from.

- Neely Hall illustration.gif (210.1 KiB) Viewed 335 times
The plans are by a Neely Hall, but this is all I have.

- Neely Hall pop up trailer cover s.jpg (56.36 KiB) Viewed 335 times