I probably need to do mine, a big job as the deck bolts go through it. I don't know how stiff/brittle walnut is vs. teak or mahogany. Even for a toe rail, teak may need steam bending. So, plastic has its appeal, and while the look isn't great, slotted aluminum sure is handy.
As far as length goes, I'd want to add a midship cleat w/ hawse anyway. So no need to be as long as the boat. Two 10-12' pieces would work.
What make is your boat, and is it on dolphin24.org? Mine's Two Moons, Pacific #250