There may be a reasonable fix. You may have scraps (cut offs from construction) laying around. I might suggest you put 2x4's or 2x6's on edge directly under each span that does not have a stud directly under the rafter. Even if you need to cut some to get enough, it will help support the weight. It still won't be as good as a double plate because it depends on shear strength of the fasteners, but it still will be an improvement.