More paper sketching and FreeCAD fiddling has yielded a triangular solar panel mount. A 3D rendering of the current design can be seen below:
Benefits of this design include the hinge being close to the ground, simplifying construction/mounting. A triangle is also stronger than a square, removing the need for cross beams. The hinge is now a simple pin or M20 screw + nut.
This design uses 2x6" lumber, since 2x4" felt a bit anemic in the long run and 2x5" isn't much cheaper. 2x7" and above is another price jump.
Potential changes include making the base of the triangular frame a bit wider.
edit: Like the previous post, here's a screenshot of the design of the adjustment bar (click for a larger view):