Concrete will actually leech the heat from your body.
Wouldn’t it just be easier to build a largish shoe box sized wooden box and throw it under the porch? Our (my daughter’s) semi feral cat with her last years kittens are currently sleeping in our old Snoopy styled doghouse that has the bottom half of the doorway closed off.
This…
If the area is as big as you say, then why not just toss an old cardboard box under there? You could slap a coat of paint on it to give it some degree of weather proofing, and it’ll easily last through the winter. Then throw a few old towels in it, seal it up, cut an ~8" opening, and voila … the perfect winter kitten bungalow.
I want to thank everyone for their great ideas - you really put a lot of thought into your suggestions.
I used a bit from everyone, actually…
I lined the walls and floor with 1" styrofoam sheets, using LocTite and a caulking gun to secure it to the crumbly concrete. So far it seems to have held - I also wedged each piece in as tightly as possible.
I didn’t realize that the “ceiling” actually extended upwards to the underside of the stairs, so I jury-rigged a roof from cardboard and styrofoam.
I wedged a 2x4 across the front to further secure the walls/floor, and then put in an extra large doormat. It’s soft, rubber bottomed, and really heavy. I have it going partly up the walls, and it really makes the nook a bit warmer and cozier.
There’s a partition now across the middle, with doorways cut in. I also bought a tension rod, but it’s too wimpy to hold up my plastic sheeting. Have to rethink my options to cover the front.
There’s no way to run electricity here, and honestly I’d be afraid to have wires going where cats could scratch/bite it. I instead have rechargeable hand warmers that I wrapped in cloth and placed near the food dishes. I’ll swap them out everyday for recharged ones.
So far MamaCat, BlackNose, Snowball, and GrayGhost have gone in to eat and check it out (don’t blame me, my niece has named all of them! 
Thanks again for all of your help!
So where are the photos?
I’m a feral cat whisperer.
I’ve had a feral colony in my backyard going on five plus years, with all of the original cats. I’ve also had numerous transient cats over the years.
[ul]
[li]Fill your shelter with straw. Not hay. Not fabric. Use dry straw. Ideally, it should be at least three-fourths full of loose straw (so they can burrow in and hide).[/li][li]Access to the shelter should be via a five-six inch diameter round opening (to prevent dogs and raccoons from getting in).[/li][/ul]
Most of my shelters are hard-side ice chests (48 quart) with the hole drilled in the end. Filled 3/4s with dry straw. This year I started wrapping each ice chest in black plastic (because water sometimes gets in along the lid seam. The shelters are wind-proof, waterproof, and dog/raccoon resistant.
One of my shelters is a luxury condo: Built on a 4x4 sized wooden pallet, it stand three feet high (with the floor a foot off of the ground), covered in tarps and plastic. Same six-inch opening for access in one corner and filled with dry straw. I also have a foot square acrylic window. On cold nights the condo is packed with cats!
You’re an awesome person! 
If they let you, please give your furry ones a scritch from me.
This sounds awesome!
Do you put food out for them? If so, how do you keep vermin away?
That cats keep the vermin away.
People love to complain about stray cats but you never hear anyone complain about a shortage of rodents.
Foamular is good stuff, I’ve created whole cat shelters out of nothing but that. Glue together a rectangular box and cut a hole in one side. You can line the bottom with peel and stick vinyl tiles to improve the durability, and add straw as well to the space. Towels are less desirable as they trap more moisture.
OK, hope this works.
There’s a picture of MamaCat (and yes, she does have a slight eye infection), and my nook-in-progress.
Thanks again for all of the help!
I saw the Foamular at Home Depot, but was unsure if it was safe in case the cats scratched at it - that’s why I used plain 1" styrofoam sheets.
Good to know that it is safe - thanks!
She’s lovely!
Could we get more pictures of the location, including inside?
I think priority should be to 1) provide insulation from the infinite heatsink that is the ground and 2) block the wind, preferably with more than one block.
You might be able to borrow a humane trap from the local animal shelter. Neutering is the only way to break the cycle.
I have a bunch of feral cats living around my house and the neighbor’s. I bought a bunch of cheap coolers, cut a cat-sized hole in two sides (so they have more than one way out), tossed an old blanket inside, and put them in the back yard.
Against all odds? That’s what ferals do.