Give yourself a hand!

Here you can see I nailed a piece of 1x2 into my work bench and then used that to stabilize the seat as I worked on it. This made assembling the seats much easier. If you look carefully you can see where the pillar (for lack of a better word) is a little longer than the seat. This is where you rasp/plane comes into play. The angle of the seat is the angle you trim the pillar down to to match the contour of the hull.
Tip: Give yourself a hand and nail a helper into your workbench during seat construction.