Pepper Ann and Babs: A Queen and Her WetNurse Need a Safe Nest

Pepper Ann gave birth to 3 kittens a week ago. I have been out of work for some time, and my financial situation has been wrought with stress. I had a pregnant cat, Liza, whom I did not have money to spay. In my community, even with assistance, it still is $60 to spay a cat. I didn't have the money.Liza had 5 kittens, whom I did not give away. A couple of tbem have since had kittens.. I am sensitive to the sad fate of kittens in shelters who miss out on life because a potential owner got a free kitten from a friend or neighbor. By keeping all the kittens birthed in my home, I am not stealing potential kitten adopters from these precious babies. However, that leads to a world of problems for me. Because of this, I have 15 cats, and 6 kittens. I am taking care of them all, and so far, thanks to friends and some community programs, their welfare hasn't suffered significantly. While taking advantage of a program that provides pet food to low income households, I was blessed with a miracle. An angel named Anne Huber arranged to have the feral cat catch and release people bring cages for my cats, snd took them en masse to the vet to be spayed and neutered. This was such a relief. Two of my cats didn't go. Liza stayed with her zecond litter of kittens who were only 10 days old, and Pepper Ann had gotten pregnant and was due any minute. Babs,anotber pregnant cat,was sent along to be aborted and spayed as she was not nearly as far along as Pepper Ann, or so I thought. Pepper Ann had her kittens 2 days later. I set up a couple of boxes for her to choose from as a birthing spot, but she chose to have them on my bed instead. Babs came to see the newborn kittens. I watched her for signs of aggression. Instead she began to clean and nuzzle them as if they were her own. Pepper Ann allowed this, and together they have bern csring for the three kittens for a week.
Pepper Ann moved from my bed to the floor besides my bed. She has made a fort of sorts, using a blanket that was hanging off my unmade bed. I need some other place for her nest. The other cats are able to access them freely, and some of them growl and hiss as they sit at the "entrance" to the fort. Also, I have no heat in the house and they are lying on a cold tile floor with only a thin blanket between it and them.
I'm trying to imagine what sort of container I might use that would have adequate room for all 5 of them, that will afford privacy, and more protection from the cold. I am willing to build something, but it would need to be made of materials on hand, or be made from dalvaged materials, or very low cost msterials. I was hoping that a mind more creative than mine would have some suggestions.
Can anyone out there give me ideas for a safe and warm nest for the last kittens any of my cats will ever have?

A closet, if you have one, with a quilt or blanket on the floor, is the best nest.  If you leave the door to the closet open just a crack, she will be able to defend her cave.   Also, she will prefer a dark place as they get old enough to open their eyes (around 10 days).   If you have a microwave and some old socks and some rice, you can fill the socks with rice and heat them in the microwave and place them around inside the nesting area.  They stay warm awhile and then you just microwave them again.  A heating pad is good too if it is covered with a couple of layers of a sheet but make sure there are places the babies can get OFF the heating pad because too warm is as bad for them as too cold.

If you dont have a closet, a large cardboard box can usually be gotten for free from any furniture store.  You can cut away part of one side to make an entrance.  If you use the socks and rice inside, and use the WHOLE box so it is covered like a den, that will keep it warmer inside.  Hot water bottles work also, and you can put those inside pillow cases.

RedTop Rescue's post came in as I was writing mine.  The only thing I have to add is:   You can also get cardboard boxes at the closest supermarket.  Egg boxes are a good size for newborns - just be sure to go early before the staff breaks down the boxes.  You can also get extra boxes to cut up to use as layers on the bottom of the box to provide more insulation from the cold floor.  Inspect them carefully for any signs of bugs.   For bedding, you could probably pick up some sheets and/or towels from a local thrift store, if one is available.