Perhaps you could get a bunch of styrofoam from somewhere cheaply and spend an afternoon breaking it into pieces and stuffing it into the hull ????
I suspect this method would have many drawbacks, the first being that a manageable chunk of styrofoam might have say, 5 pounds positive flotation. That means at least 750 trips to the bottom, placing the foam and then returning to the surface for more. Not to mention the time involved in cutting up the foam initially and then the inevitable escape of many bits of styrofoam and the neccessary recovery of that foam (everybody involved certainly wanting to be good stewards of the environment and all