Snag city!!!! I agree with Mac Daddy 100%. It's gonna be difficult to keep that vertical. You can try using a length of schedule 40 capped at both ends instead of bottles. Shouldn't have to worry about that collapsing from the pressure, but it will just make it that much harder to sink. If you are wanting a tall brush pile, try using bamboo in your buckets and then fill with concrete. Drill holes in the lower segments of the boo so they will fill with water, and leave some of the top ones alone. That will help keep it vertical. I like this type of structure because you can make them as tall, short, thin, or thick as you want. I like to trim all the branches and just leave the stalks for areas that I plan to spiderrig, or to place near a larger pile. This will usually hold a thumper or two. Just my two cents...