I think the cushion might not have the correct pattern? When I did one with wood I had to make a pattern using paper bags.
The approach depends on whether it is going to be a hard shelf or soft cover. For a hard cover, remove the seat backs and make a cardboard template at the same height as the intended shelf (the outline will be different than seat cushion or seat back). For a soft cover like carpeting, use wide wrapping paper to trace the template and add .5 inch to the outer edges so it can be tucked under the trim.
It is 1/2" MDF, I will just trace it out. I have the plastic trim on the side out, I might use them as a template.