@pmcnyc I don’t know of any other way to do that besides the unavailable times feature. The other thing you could do is add an individual time of a certain class that you want to take to your cart. When you add a time to your cart, then you can go back to “build schedule” and it should only give you options that include that specific class and time. Basically, just start with a couple classes with ideal times and professors, and then you can fill the rest in from there.
And yes, I do have my schedule done. It might change slightly if a class fills up before I register though. I have a couple that are probably going to be close.