I'm sorry the rescue groups won't help. If I lived closer, I'd come over & help. (I've rescued many kitties over the last 7 years.)
Would you be able to fit a humane trap under there? Some of them are kinda big, but since kitty is so little, you might be able to use one designed to trap squirrels. You can see if one of the rescue groups would be willing to loan you one, or you can see if your local Home Depot, etc. has them.
And good luck! I hope the little one decides to come out soon!