I have the same boat and plan on doing the same thing! My idea is to drill out through the side of the hull just below the liner to attach the cabinet above the counter. I haven't finalized the design yet but i don't want to encroach on the counter top so the width will be quite narrow. Also I will be installing narrow shelfs along both sides above the benches leading to the front birth. I plan to build a more vertical backrest which will then support this shelf. Through hull bolts will also be needed for this shelf.
If you want to send me a private message we can discuss our options in more detail.
Andre