Installing a harness

Sorry guys I didn't clarify but mine did originally have shoulder belts but the elastic that holds the shoulder part up is shot letting it dangle. Plus they just aren't comfortable because it catches me across the neck. That's why I'd like to replace them with a harness. Been considering a 4 point roll bar just to have something to mount them too but it's a lot of work for something that only gets raced once every 2 or 3 yrs. At the moment I haven't been able to think of any other way to mount them. Just thought I'd throw it out in case anyone has figured out a fairly easy way. Appreciate all the replies