Answer: Rather than try to match the slant of the roof, perhaps you could make brackets to hold shelves to hold normal flat bottomed boxes all the same size; the shelves could sit level with each other at the bottom of the windows -- the actual bottom of the windows would be hidden from front view so it would not be noticeable that they are all a bit high. The main reason I would suggest this is that the plants' roots need the maximum possible amount of soil and an angled box would limit the soil space. If some plants have more root room than others they will not grow symmetrically. Alternatively, you might be able to find a woodworker who would make you some. I have never seen plans off the rack for what you are describing, probably because there are so many possible roof angles.
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