Wow that looks familiar! Looking good.No need for anything that heavy on the inside of the lid. Just something to take up that gaps and make for a even bond with the new core material. So a few ounces of mat with a filler will do. The strength needs to be what you put on top of the core material that will become the bottom of the lid. Keep in mind that portion of the lid will be placed in tension when a load is placed on top of the lid.Progress is looking good so far. I sanded my non-skid down a bit. I painted my decks with Petit easypoxy with some non skid added to it where I wanted nonskid. Held up well while I had the boat.