Something along the lines of what you had mentioned Dan. Teak core, foam float, fiberglass covered.
The "football" float. A prototype made by a friend some years ago based on sufboards. Very light with ~ 18 lbs of lift. I don't recall how deep it was taken to test the crush effects, but I think it was somewhere around 200'.
I like to use this float with a 60' bungee on paddies or weed lines for Yellowtail and Mahi. Obviously not for bigger fish, but very nice for making investigatory drops. Makes it quick and easy getting in and out of the boat.
The infamous "kelp bombs"?