http://www.kinze.com/binaries/pdf/op...MECHANICAL.pdf
Perhaps you could adapt one of the press wheel springs to work for your application?? They have a hook at the one end and then a nut at the other so you could thread your rod into it. These are available at John Deere dealers or White dealers. They are used in lots of places on corn planters - different lengths and sizes. Just a shot in the dark?