My favorite fluffy backing for a quilt is fleece.

I love not only the softness, but that batting is not required. The fleece has a certain 'grab' to the fabric - though basting is still necessary, it can be done much farther apart. It does have quite a bit of give - it wants to stretch, so I don't tape the it down before basting.

Finished - for Addison's 10th birthday. Just a little late.

She knew it was coming though.