Go to the local gun store or shotgun range and buy some lead shot used to reload shotgun shells. It comes in 25-pound heavy-canvas bags. They're great for holding down parts being glued in the middle of large flat surfaces. Be sure to put a piece of plastic sheet between the part and the bag so the canvas doesn't get glued to your kayak. Its not cheap, new shot will cost about $16 a bag. If the range sells reclaimed shot it will be about half the cost of new.