Good point. Actually, we have a favorite Fettuccine Alfredo recipe that has you making the sauce over the pot of boiling water, too, and we avoid the mess by using a large, rectangular glass baking dish on top of the round boiling pasta pot. The shape of the dish allows it to sit securely on the pot, but there is space on either side of the dish to allow steam to escape without causing a boil over.
Another option is to add some oil to the boiling water, which raises the boiling point quite a few degrees. But you’re right though, you would certainly not wish to use a tight-fitting round plate that seals in the steam only until the explosive boil-over.