you can get some ratchet straps are walmart or lowes. they are stronger and don't stretch like elastic or bungee cords. If you're not comfortable with that, you can pick up some cheap C clamps at the dollar store or walmart. Put 6 or 8 of those on there, and that would work well also. Or, get a piece of shelving or plywood big enough to cover the lid, and place that on top of the tub, then set a few bricks or a cinder block on it. These are al pretty crude methods, but they'll work.