somewhere I saw an article by a gentleman who made these seats regularly. He had made a jig for his router, and I am trying to recollect how it was all set up, but pretty much the router base was fastened to a round bottom plate which rode in a jig shaped like the waves which you want to rout into the seat. Not very helpful I know. Sorry. I just cant recall where I saw it. No doubt a magazine, but I have hundreds of them and don't want to go thru them all. Anyway I hope this gives you an idea and you can run with it from there.
He is no fool who gives what he cannot keep to gain what he cannot lose
If you cannot stand behind our troops, (and I understand, no judgement here), then please stand in front of them, remember, they're willing to stand in front of you!