Google the term "rotational molding" and you may uncover some useful information after a bit of reading.
What you are considering is an established technique commonly with thermoplastics. A split metal mold has a measured amount of plastic granules put inside and it is closed and then rotated around two axes in a sort of gimbal device and is heated. The plastic granules soft and fuse and coat the inside of the mold evenly all over because the rotation is around two axes.
The mold is cooled and opened and the finished object removed.