That is similar to how I did things when it happened. I literally stepped out of the room for a *moment*. Pistol was out of the cage talking to Fatty, so I figured it would be fine and she would continue to do so. She was not only gone when I came back in the room, but greusomely so. If I had to be in a situation like yours again with only one room, I think I would look into those hanging velcro bird net barriers to put up around the rat cages. Edit for clarity: the rats were in their cages, Pistol, even not being allowed over that side of the room at all, flew over to the cage in the short time I was not supervising her.
Maybe something like the above product, but large enough to completely block access(or slow access long enough to intervene). The hard lesson I have since learned is that a threat never seems "real" until it is *too* real.
And I am a fellow rat lover. After that happened I had to rehome all of mine and have some rat free time to let the intensity fade a bit. I only recently have gotten to a point where I could have rats again. It wasn't their fault-they were just being rats, but I wasn't able to separate blame from them for what happened. The 2 boys we adopted just recently are lovely animals, but they are NOT trusted around birds even for a second.
I'm sorry that that ended up happening to you. To say that that was a horrible experience is a major understatement. I'm looking into getting something like the product you described now, but I just have to hang it close enough that Callum can't get to the cage, but far enough away that my rats can't pull it through and chew it up/ tear it up. I'll also have to see if my apartment would allow it. To my knowledge, my apartment has a rule against hanging bedsheets on the walls. If anything, I'll attempt to talk to my apartment complex and see if an exception can be made for their and my bird's safety.
Thankfully, I have one of those huge Double Critter Nation cage for my rats, so it's only one cage. I actually got a stand to give Callum his own space. He would constantly try to land on the rat cage and I'd take him off. Now with the stand, he's got his own space to sit on and doesn't feel as big of a need to sit on it. For now, whenever I leave the room, Callum is usually on my shoulder or is in his cage, so I don't leave the opportunity for him.
I'm glad you rescued two new boys

I brought home two brothers a few months back and I'm glad I did, I lost one of my rats recently. The rats act as my unofficial ESA's, but to actually get that label on them would be difficult since they are so short lived, unfortunately
I really sound like I'm making a ton of excuses and I'm not denying that I am, and I really don't mean to sound like I don't care about Callum and my rats. I really do, but I just have to find a solution my apartment will be okay with. And since they can come into my apartment whenever they want, I can't necessarily sneak something by them. I'll try to research more solutions to this Callum-rat issue when I get home (I'm currently at college and have to go to class).
Any other suggestions would be appreciated.