the bottom wall needs a diagonal brace to its top from the bottom of the middle stud.
I'd put the top blue bearer as close to the top as practical, and use a double one there.
I'd put something across between the bottom plates to stop them from splaying, could just be another 2x6 and it could go in the bottom corner if it's well attached (glue and screw).
Attaching the tops of the verticals, end nail or screw from above is easiest.
2x4" for the joists seems undersized.
treating it as a floor it wants 2x6 but 7 of them would be enough.
The slope helps as the plywood will be able to carry half of the load.
so maybe the 2x4 is actually strong enough (at what I assume is intended to be 16" spacing in your drawing)
The top plates sloped green ones) should probably be double or you could notch the studs (vertical green ones) and install the top 2x6 with the narrow side up, that would simplift fixing at the top as you can just screw or nail straight through the side.
by notching you get the option to run the studs past the top plate to connect to the joists, this will help strengthen the sides against splaying apart.
you probably need more studs (vertical green bits) one for every two joists at-least.
This should go under the plywood. (so the plywood goes between the green and the blue)