i use blue barrels that came from my uncles carpet cleaning business. Dont know where else you can find those. We also used old tires, tables, a sofa, palets, doors, plywood, we had old shelves that my friend got from models cuz they were getting rid of them, we even used a small boat.
We also used dirt. We also tried making bunkers out of planks we made frames out of then stapled chicken coop wire to it and my frined had a material that we covered it with. It didnt work out, the material wasnt strong enough. It was a cool idea though we even had a dorito.
Heres the images
http://i37.photobucket.com/albums/e6...ballfield4.jpg
http://i37.photobucket.com/albums/e6...ballfield3.jpg
http://i37.photobucket.com/albums/e6...ballfield2.jpg
http://i37.photobucket.com/albums/e6...ballfield1.jpg