how do I do this?

 From:  Michael Gibson
1977.4 In reply to 1977.1 
Otherwise to build it more directly in place, you'll want to split those curves into 2 pieces to use as rails for a 2-rail sweep like Pilou shows. Shelling may have some difficulty processing a sweep like that, you may have to create an interior surface manually by doing another sweep for the inside part instead of using Shell if you go that method and shell has problems with it.

- Michael