I built one as follows.
I have provided a crude pic below. Basically I spaced the floor joist so that the drums would wedge them selfs in between and not touch the floor decking putting the load on the joist. Rope was only needed in case of high wind, otherwise they stay in place with out it. all put together with decking screws
RT,
I used the same "pinch" method as you on my floating deck. I didn't use any ropes or attachment methods. The drums are solid, and have never moved.
I started with 10 drums on my 8x16 deck. The 1000# deck floated a bit low, and the 2x12 frame would touch the water when 2 of us stood on one corner. I added 4 more drums for a total of 14, and that fixed it. The drums were free, so I added 4 more for a total of 20. I estimate it would take ~7000# to sink it.
