your first idea with the hook where the back glass goes would work fine. Your straps all the way up to the front 2x4 would works as well, but I would go with the hooks.If that's too much force on the front 2 x 4, I could lay two 2 x 4 lengthwise inside the top and use straps to attach those to the upper rear 2 x 4
I bet you could go wander around harbor freight or home depot and find some hooks that you could modify to work.
I was also thinking maybe some 2" wide flat stock bent with a bfh and a vise would make something that would work out perfectly. Pad it like ryoken said and good to go.

