I'd think about adding some sort of lightweight foam to the side panels. Maybe and inch thick and 6-8 feet long centered around the cockpit of the boat. Bevel the bottom of the foam to match the existing bottom angle, fair the front and rear edges into the existing side panels, probably bring the foam up to deck height. Now the only question is what kind of foam. You want something that won't ding easily, yet is light enough to add flotation. Even if you could sheath it with 'glass and epoxy there would be a weight penalty. I don't think you'd need to add much to substantially increase the primary stability. For instance, I'd bet that two inch thickness on each side would be way too much.