I have a feral that I've been caring for for over two years now. She has lived in my neighborhood for 5-7 years. Lucky for us, we live in Florida. So we are having our coldest weather of the year right now, which has been in the 40's at night. We have the occasional 30 degree nights. Just recently, in fact last night was only the second night we've done this, we have got Arwen (our feral) to sleep in our garage. Sometimes we leave the garage cracked so she can come in and out, but twice now, I have shut the door real quick with her inside. She mewed a little bit, but she slept in there all night. If she's not in the garage, she will sleep by my front door. I found a nice warming bed at Petco, it only warms to 102, their body temperature, and it only gets warm as they lay on it. It is very low watt so you can safely leave it plugged in.
If I were you I would make a feral bed/box, I have seen how to do it on this forum. It's basically a styrofoam container that is inside a rubbermaid box with some insulation in between the rubbermaid and the styrofoam, then some straw or hay inside (I'm not sure which one but look it up because one is desirable over the other and I can't remember which it is). I tried to do a modified version of this for Arwen, she wouldn't have anything to do with it. I don't think she liked the straw or hay bedding. Maybe just a nice warm blanket in there also would be something the cat would like. Arwen does not like to be trapped into something, so if it doesn't have a front and back door, she won't go into it. If you could make some sort of box/enclosure, near your door where the kitty sleeps. Even better would be a garage or shed with a heating pad. I am completely unfamiliar with those kinds of temperatures you are describing, but I'm thinking that is very deadly temperatures for any animal outside. If you can get Scruffy to come inside tonight, just put her in a bathroom, laundry room, etc, with a litter box, and even though she might not be happy about it, she will be safe. That is what I would do. Good luck, what a big heart you have to take care of her and her babies.