Do not use a cinder block under the jack; the pressure point can shatter the block. The jack will be more stable with the least amount of extension possible, so many people block under them , use wood , a block of 8X8 , would be preferred , but gluing and screwing 2X4 pieces every level across the one below to your desired height with a metal contact plate for the jack will work, you may have to wedge the stack to make it stable on the ground depending on the slope.
Remember that the plastic wheel chocks can crumble, so use wooden chocks or the solid rubber ones