"The thing is, what shape ought it to be?"

Same exact shape as the original. Without the cuts/steps, of course.

The nut won't loosen up much if the cotter pin is set in correctly.

If you go the C-clamp route, you need a very heavy duty C-clap and optimally one with a fine thread. You will also need some sort of thick metal spacer/cylinder.

Again, read Sheldon's page.

http://www.sheldonbrown.com/cotters.html

http://oldbikeblog.blogspot.com/2008...otters_17.html