Alright, i will let you in on my secret. I am an electrician, and we get a lot of wooden spools of all sizes. ( i have one in the back of my truck now that is 5 feet around).
I sank 10 of those in a gravel pond i fish. 5 with no add ons and 5 with x-mas trees or stick piles cemented in the center. Out of everything i have tried these by far hold more fish than any other structure. I am getting a trailer to haul two 10 foot around by 4 feet high spools for this springs fishing. Go to any graybar or platt and they are more than happy to give you all of there spools. I went to graybar in eugene today and they had 20 or more spools of all different sizes. Very easy to sink in the water without cement too.