Winter Shelter for Ferals

I am taking care of three feral kittens (7 months+).  I've gotten two out of
the three spayed/neutered.....the third is pretty crafty!     Of course, I
am worried about the coming winter and I'd like to know what others have
done to protect their ferals.  I live in the city, so it's very important
that the shelter not be conspicuous to passers-by who might not appreciate
what I am trying to do for these cats.  I found a site with a styrofoam
shelter that seems do-able.  I am not handy enough to build the one on the
feral cat page.  I look forward to your responses!  Robin