When going over bumps while using her power wheelchair, this Client can knock her left shoulder blade against the back of the chair since she sits slightly inclined. This causes her back to dislocate which is very painful and uncomfortable. 

Your task is to design something that would reduce the amount shock on her back. It must be easy to install and simple to use. It should also be waterproof and comfortable against skin where applicable. All warranty requirements for the wheelchair must also be respected (no major modifications like drilling can be done).