I am happy that you noticed the lines. I bought a metal seat frame and had it custom welded with supports to mount it. I made the frame for the seat from 1/2” plywood. I made multiple cuts in the plywood to bend it and epoxied it to fill the gaps.
I formed the seat from layers of foam with an electric carving knife. I was lucky to have an upholstery shop in town that could finish the seat for a reasonable price. I really wanted to keep it a two seater as opposed to some of the cafe racer builds.
The main reason for the lines is the messenger bags, they cover the frame and tend to straighten the look. I made the messenger bags from leather by hand. The bike is now for sale.