Hi Steven,
Bent stiff wire is adequate, it is usually a good idea to have a dog leg bent into one end with the other end bent at right angels.
The only problem with the bent wire is a lack of fine adjustment, you might be alright because you haven't installed your servo yet.
With the bent wire idea I am assuming that you have complete access to the rudder arm once the model is finished. If that is not the case I would tend to go for an aircraft type of linkage these have the added benefit of adjustability.
I hope this helps.
Cheers,
Stephen.