You don't say what type you're talking about.
One word of warning - check the flight manual carefully - many modern helicopters have crash-worthy, downward-stroking, seats which prohibit placing anything underneath them!!!
If your type is is not subject to such a prohibition then i've seen people use sand-bags, lead ballast-weights (sometimes simple gym weights) etc. It's very important to make sure that they are properly secured otherwise they can easily be a loose article risk to the controls.
OH