You could find a couple of squarish rocks and set them on either side of the pump, then set a larger flat rock on top of the pump.
One note about pumps, though. If you have fish, you don't want to set the pump on the bottom. If something goes wrong, the pump can suck out all of the water and leave your pond empty. I suggest setting it high enough that if it pumps out all of the water it can, there is still enough for the fish to survive.