I have also built a floating island out of a variety of materials. Mine is made out of foam and bio mass materials. It has grasses and plants growing on it and it is in its 3rd year and still going strong. I see ducks frogs and turtles on it regularly.

Total cost for this was about $32 for a 5'x 6' island. This type allows the plants roots to grow through the bottom of the island and the fish hide in them and it reduces the nutrient load in the pond by the plants using it as opposed to it going into algae.

If you like I can take some pics and send them to you. Let me know what you might like.

---------------------------------
1/10 - 1/4 acre pond plus 16 ft deep/ Plus 40 ft by 20 ft by 6 ft deep koi and fathead minnow pond next to it. Upstate NY