I cosplayed and made Rachel's costume last year and the way I made my skirt was make a base hoop skirt, then make the white underskirt ruffle and black overskirt seperately and NOT attached to each other. The skirt still lays out into a perfect circle.
Since you don't want to put hoop in your skirt, I can't figure out another way you can make it poofy, but I will suggest maybe trying my method as the white ruffle and black over skirt hides the hoop skirt and yet it still lays out perfectly circular
You can view my gallery for reference ~ http://www.cosplay.com/costume/321683/
London Flim and Comic Con 2011
Rachel Alucard from BlazBlue: Calamity Trigger
Riccotta Elmar from Dog Days
Nanoha Takamachi from Magical Record Lyrical Nanoha Force
Madoka Kaname from Puella Magi Madoka Magica