I think it's so kool that you are taking care of the feral kitties!
How many are there?I am a big kitty kat lover..heehee
Anyway,you were wondering about what you could set up for them outside to sleep in?
The cat lovers group i belong to someone had a question like this once.
I cant remember word for word..But here is an idea.
Pick up one of those larger rubbermaid containers w/lids.
Cut a hole in the side of it ,big enough for the cats to get in and out of.
They are waterproof and will keep them ALOT warmer.
Also put something like hay/straw in the bottom and cover with some old blankets.
If you dont think that would work.I had just one cat outside here last yr that i cared for.
Not sure what size box you would need but
I used a medium sized box w/lid that i attached ,so it would be half closed to keep warmth in..Then covered it in heavy duty plastic.
Then i put some old heavy towels in there for her to lay on.
If at all possible try and just put food out for them a few times a day.
Dont leave it...
Otherwise you wont ever get rid of the other lil creatures lookin for food!
I will ask around at my catlovers group to try and get you some better ideas!