[Shadowkiller]

Quick and nasty design:

A couple of big foam floats, with mast and flag to find it later, chain for the first few metres so it doesn't get cut off, then rope down to a tyre filled with concrete.

BUT: Private FADs are considered ocean dumping in some countries, Australia included. Check with your local waterways/boating authority to find out if its legal or not. Usually you will need to stay away from shipping channels, and have a radar reflector so ships can see it.