find a tire shop that sells tractor tires and/or deisel tires. these larger tires create much larger structure, although harder to handle. if you cant find a store such as this near you, find a trucking company. They should give you as many big truck tires as you could possibly need. you can stack them or pyramid them, just use your imagination!!! good luck, works great for me. also the tractor tires are a much harder rubber and your hooks won't sink into them as easily as car/diesel tires but once they do, your not as likely to pull them out.

Keven