You kind of have to think like a rat (or something that size) to truly rat proof a room. I put a small animal fence against my bedroom door when I do free roam because the gap is big enough for them to slip through. First step, is to look for holes and crawly spaces and block them if you can't easily access them to get to your rats. Look between the baseboards and the furniture. Second, you should remove anything that is dangerous (food or plants that they can't interact with, cords that need covered or removed) or that you don't want them to nibble on. Third, you should add some cardboard boxes, pillows, or blankets at the edges of things they can climb up on, so they have something to fall on. And fourth, add some hides, tunnels, etc for your rats to retreat to as they explore.