Base base and base

For the bottom part of the cocoon i want to maintain a circular shape for the bean bag to sit in. When thinking of this and what products look similar I thought of the soft shapes that i used to see at Playzone etc.  I also thought of sofa cushions and how they maintain the shape. This is done using foam (not extremely dense but enough to keep the shape). I will use this foam to fill out the shape in the base but I am prepared to have to add some sort of shape to help maintain the circular shape.