Yep. Dirt. Lots of it.
Try and build a mound twice as long as you thing you'd need, at least 6 ft. high, and deep enough to withstand erosion and the barrage of projectiles (approximately 4 to 5 ft wide at the top, obviously much wider at the base. If possible, build it so you can somehow tamp it down and aid the settling of the materials.
Some folks have 'terraced' rail road ties to aid the holding power until the pile settles. Just like concrete, it helps to use varying size and types of soil, some clay, some larger media, and even some very fine particles to aid in the settling process.
"The fat lady is standing still. She's taken in a very deep breath. She's leaning forward just about to mouth the initial word..."