I was thinking of designing one myself for a long time. I chose a well-known shape (the infamous Skull & Bones) which proved to be very practical and accommodating. There are enough contact points to anchor the cords in several different ways, depending on the occasion and large enough for two headphone pairs to attach on. You can find it here.
The picture with the headphones in the eyes looks really cool, although the one with two headphones rolled looks like a ball of yarn... but the device is cool.
Thank you. I agree that attaching two pairs makes it a rubber band ball but it is doable and still much better than tangling in my bag out of control!