It is the water that needs structure not the bottom. What does that mean? When thinking about fish attractors you have to think like fish do - you have to think about the water area in 3-D. Under most circumstances you want the items to cover the water column from at most 3 to 12 feet in depth.

In a 1/3 acre pond other than the shore items you need at most 3 pieces of structure. So make 3 of Brettski's PVC trees but have the main pipe 12 to 15 feet long but only put cross pieces in the top 8 feet. Set them on the bottom , one in each of in the deepest areas. They will not slide if there is no deeper place to go. That will give you 3 deep areas to fish.