I don't think you need diffused light to make contact prints. The purpose of diffuse light in enlarging is to eliminate the Callier effect. You don't have that effect in contact printing because the negative and paper are in contact with one another. This is not to say that you can't use diffuse light, just that it doesn't really accomplish anything over a condenser enlarger, so long as you raise the condenser head far enough to be sure you are getting even light across the negative.