I've seen some, on racks, of big utility trucks that would fit the need. They are thick and large but not cheap. Some of them are close to hundred dollars per pad, best I remember. sorry I don't remember any brands but they are on the net. I carry 2x2x14 inch pieces of treated wood. they are as large as that compartment will hold. They do well except in soft soil etc I've had them break up under the weight of the MH. The large ones I saw on the utility vehicles are probably the way to go, if I wasn't so cheap.
Near Austin, we camped, at a Resort two years ago. The ground looked dry and had large rocks in the spaces. With the pieces of wood under the jacks, they still sunk over a foot into the ground. Seems there is a layer of rock underneath and water flows over it and the soil and rock look dry over it. We had to move the coach slightly and they brought out large pieces, of thick plywood and big boards to put the jacks on. You can't get too big or too solid in jack pads. You just might not be able to store them. :)