Either foam or plywood or balsa. Regardless, the proper repair requires removal of the skin, recore and reglass. If you remove the bottom skin, the repair doesn't have to look pretty.It seems that the core of the panel has deteriorated. I don't know what the core is made of, perhaps foam.
Thank you for your suggestion. Beneteau is contacting France to see if they have the part. If not, I guess that I'll have to have it fabricated. The floor of the cockpit may have to be resurfaced to have it all match.Did you contact Beneteau to see if they stock a replacement for anywhere near a reasonable cost? Unless you love doing glass work, it might be worth a shot. The best way to repair it otherwise is going to be to cut out the bottom of the panel, remove the core, and replace with either balsa core or foam. It's not going to provide a lot of strength so a couple of stiffening ribs glassed into the bottom panel during reassembly would be worthwhile if there is clearance over the steering cables.