Thought I'd share what has been a useful way to increase storage in the cabin of Wren, our '87 16 II. This also holds the Porta Potti securely and creates a level floor, which we really like.
I used my daughter's sidewalk chalk and coated the plastic edges of the bunks between the companionway and the forward step up (where the bunks meet in a point).
Next, I pressed a piece of 1/2" plywood down on the chalked edges and stood on it, lifted it carefully and presto, chalk lines. I used hot pink. She got over it.
Cut it following the chalk lines, then measured the width of our plastic bucket porta pottie and made a cut to accommodate it, then cut the remaining piece into two pieces. Drilled 1" finger holes to lift the panels, sanded everything, primed and painted the three boards.
Then screwed in corner braces that extend like a flange. I used lots of these, and beefed them up with bigger wood screws. Wrapped the braces in electrical tape, and done.