Designing an ottoman for a modern sectional sofa is not very different from designing one for any other type of sofa. The main thing to bear in mind is that the ottoman should match the sofa, so as you design, plan to get material to matches that of your modern sectional. Another tip would be to make the ottoman the same size as one of the cushions on your sectional sofa. If it is a corner sofa then make it the same size as the corner square. If you have an ottoman the same height, shape and size as a cushion, you can more easily turn the couch into a functional bed.