We were very pleased when we cut into the space behind the seat cushions between the liner and the hull. Foam blocks were fiberglassed into place to form shelves. The openings were lined with automotive style edge trim. We ended up with four "hidden" and yet very useful cubbies.
We also added shelves outboard above the cushions. The factory brochure showed enclosed lockers in that space, so we copied the factory idea. We removed the fabric covered panels, and the teak trim. Two six foot long by 12" high cabinets were installed. The fiddle rail was discarded. A new teak sea rail is mounted at the upper edge of the cabinets about 10" from the overhead, so we still have a place to store decorative and useful items visible from the cabin.
I've read (in this forum) that the box that forms the companionway step must not be compromised by cutting a doorway into the space because it is a structural member that should not be weakened. I have also seen pictures of doors being cut into the space. Make your own decision.