I honestly used regular the-cheap-wall fabric and it was well received. Granted, this was back when I didn't think fabric mattered... I think if you're adventurous that a thin vinyl/pleather may be good. But in all honesty... even just the regular old cotton fabric got VERY HOT. so be aware that you're gonna bake.
More than that fabric, please be sure to do the collar well... nothing makes Haru cry more than a collar made with a white boa. ~_~;;