Karen McTavish has been doing this for years, you should google her and look at her work then purchase her books one comes with a DVD.
I will do doing one soon with batting, fleece and wide felt, I will use bright colored fleece and felt as once it is overlaid with bastise the brights will become pastels. You can also use a Toile of print fabric as your shadow piece, Karen shows it all in her books and on her website.