I assume that your boat is a 1963-70 model? Those were the ones that were completely open bow to stern, with a narrow deck around the sides, a seat/tank in the bow (flotation), a seat/tank in the stern (flotation), a cross seat that the mast steps through, and removeable wooden seats between the stern tank and the cross seat (mast partner). That was what mine (1969 model) had.
I added a stowage compartment between the forward flotation tank/seat and the mast partner ("seat" that the mast steps through). I'm not sure what I may have for pictures...... but I'll look! Basically, I glassed in a platform across the boat sitting on the "box-beam" that contects the CB trunk to the forward flotation tank/seat, then glassed in a bulkhead from that platform up to the level of the mast partner, this was cut to conform to the inside of the hull and glassed to the inside of the hull. The top was supported by a frame that was screwed and glued to the aft side of the forward flotation tank/seat about 2" or so below the top and spanned the top of the created box. I permanantly installed a triangle piece of plywood at each side of the top to square up the top opening and had 2 hatches that covered the compartment. I never did get the box totally watertight from above (so it did tend to leak when it rained!), it was tight from below, but I should have worked a bit more on designing a leak-proof cover!
One side benefit to this compartment was that because the metal "frame" that O'DAY originally molded into the mast partner had rusted and was no longer firmly attached to the underside of that mast partner, my boat would actually squeeze together while sailing due to the pull of the windward shroud and the boat twisted slightly from bow to stern rue to the lever force of the mast. After adding the stowage compartment.... it reduced the twisting and pretty much eliminated the squeezing inward from hte shrouds.
The forward flotation tank/seat is filled with a pretty solid chunkl of foam, the aft flotation tank/seat is also full of foam (styrofoam!) There is foam up under the gunwales almost all the way around the boat.
I'll see what other pics I have, or at least find/make some sketches.