I built mine just as Bill described, except they are not foam filled. They have been in the water for about 3 to 4 years. Flotation has never been a problem, but balancing the plant life is.
I have a predominant plant that is slowly squeezing everything else out. I made 3, 1 meter square sections and bound them into an L shape, the ever aggressive plant has cloned a forth section, almost a perfect square and is now filling in the center.
I let the island free float and it finally anchored itself. It is now closer to a peninsula than an island.
