Good question! This is a project that I have been considering for some time. I do not believe that there is any "right" solution. However, my plan is fairly straightforward.
The V-berth is quite wide at its head and some of this area can be sacrificed without damaging the berth's comfort. By assuming a standard double or Queen width berth, one leaves a triangle shaped footprint on either side of the boat. Each triangle is formed by the edge of the standard matress, the bulkhead and the boat's hull. Because of the taper of the hull, as you project these triangles vertically towards the deck their areas increase quite nicely. They also terminate at the deck.
My plan is to use your "hockeysticks and marine ply" to construct a locker on each side of this area. One could either have them open by tipping outwards, or could simply cut openings in the ply like a cave locker. I am still trying to decide if I prefer bins or shelves so it looks like this will be a spring project. I am also trying to decide how to handle the forward termination of the flat panel. If you follow it up to the deck then you have a long and fiddly curve to fit along with a lot of dead space behind, but if you terminate it with a transverse panel the installation is not as tidy. I am leaning towards a hybrid at the moment -- follow the curve up to about 8" elevation and then truncate.
When planning this project, don't forget about the thickness of the cushion in your calculations -- it will dramatically reduce the area in which you can cut appropriate openings.
One other suggestion I will give you is applicable to all of these interior projects: actually get on board the boat and try out a few ideas in cardboard. I find that I often overestimate size and space on the Contessa. For example, the distance from the hard V-berth surface to the deck is only 19". Subtract a cushion and perhaps bedding and you have relatively little space left for the openings of your locker.
(Aside: Another project that I have abandonnned involved replacing one of the quarterberths with a dinette such as can be found on boats such as the Tanzer 22. Unfortunately, when I actually got into the boat and played with the dimensions, the proximity of one's head and the sloping surface adjacent to the cockpit pushed the aft seat so far forward that the only way to make the design work would be to cut out the sink cabinetry. Too much surgery I think.)
Please keep us posted as to your progress -- I will be very curious to see your solution.
Christopher