Hey! I just finished a sofa just like that! In reality, NO. There is no way to do that sofa without seams. The top part of the cushion will need to be the exact shape of that seat, allowing for seams of course (my standard is a half inch.) I use a seam puck to trace my cushion to get the exact shape with my seam allowance.
Sometimes I trace this on paper first (you’ll need like butcher block of plotting paper that is big enough). You can get yourself a seam puck or you can use a sewing machine bobbin, which just so happens to be a half inch from center to the edge. Just stick your pen in the middle and roll along the cushion edge. You’ll sew the top of your box to your side, which, if possible, you should cut in one continuous length. If you don’t have the length and need to sew a few pieces together, keep the longest section in the front and sew smaller lengths to the sides to hide the seams where you can.
You can get real-time help from me in my private facebook group. Once you join you can tag me in a post and I can help you along the way! This group is free to join and participate in. You can find it here:
https://www.facebook.com/groups/1379461489191063
Buy the Seam puck here:
https://bealsupholstery.com/collections/all