My 78 Hobie 14 has a soft spot near where you described. I have considered purchasing a 4 or 5 inch hatch cover w/ flange. I would then cut a 4 or 5 inch hole in the soft spot, remove the hull, turn it upside down on saw horses, and reinforce the area around the hole from the inside with fiber glass cloth and resin. Then mount the hatch cover.
<br> I don't know if this is the best, easiest or even proper form of addressing the problem. You should get a second opinion. <br><br>