I'm always surprised at how much weight it takes to get stuff down. I had a pallet stake bed refuse to sink with 50 lbs on it and I had to add another 25 while it was in the water. The other was a 5 gallon buck full of concrete with a mass of bamboo. It sank about half way and just set out in the lake. If finally sank, but it took awhile.

Thanks for the pics. It looks like you'll be in good shape with those.