I have an idea for one and it is to use plastic pallets upside down so that it have walls. I have a couple in my pond being weighted down with stones for FHM. They are 4' square pallets with pre existing holes. They are pretty expensive to buy. so If I had to buy them I wouldnt use them. But being that I already have them and they float very well I will make my attempt with those.

3rd acre fertilized fed and aerated