I was just going by the external "glass keel on my 40 foot fishing boat hull. And its quite a bit bigger than yours, still not 20 gallons, and I filled it with epoxy and cabosill. It was a big stinkin chunk of lumber, kinda like the originall description. And who's to say what was put in the hull of a 1977 asian built boat, certainly not any of us, unless we have actually checked ourselves, which we have not. It may well be hollow. If it is, personally, I would fill it with concrete, leaving a couple of sump stations for pumps.