Plugin help

I am running a volunteer group that needs to sign up for 4 main functions. I am wanting to provide then the choice of 4hr shift or 8hr shift in each function. Each person can ONLY sign up for 1 function each day. What is the best way to set this up? I have set up a service 4hr and a service 8hr. When I try and set up the time settings I am kind of at a loss. I want then to be able to have the volunteer either select the 4hr block or the 8hr block. How can this be accomplished?