When I was working on research vessels, the scientists would sometimes use Railroad tires. Those steel tires that you see the box cars rolling on. You need to find out where a railcar salvage or refit yard is at and see if you can get one or two. They make great mushroom anchors. These are used by the scientists as throw away anchors. The Science package is connected with a timed or sonar activated release and over the side it goes. We come back later and pick up the sci package sans the railroad tire.
But they do make great anchors and in the mud would hold a 32 ft'r with no problem.
But the state of Washington is antsy about what is put in their waters.