Outdoor Shelter Suggestions?

We have a stray in our neighborhood who we are caring for. You cannot
get near him anyway :-( Unfortunately we are unable to take him in at
this time, due to finances and our other 5 kitties, one who was just Dx
with CRF.

It seems "Whitey" is living in a cement drain at the rear of our
property. Where he goes when it rains is anyone's best guess.
Fortunately living in Georgia, the winter has not been bad. Whitey gets
water and dry food outside the door daily. I assembled a cardboard box
with some towels inside to see if the cat would use this as shelter.
The oppusums have gone in to check it out, and Whitey will go in it
when I place the food inside, but he will not sleep there. Even in the
pouring rain :-(

For shelter..in that Dh is not a carpenter, we would need to invest up
to $100. Have done a little research online and would of course take
any suggestions here :-)
Is there a way to get Whitey to accept our offer of a dry place to
sleep if we make the investment?