Is there a line where muck turns to gravel/rock at any depth?
If not, you need some shallow, somewhere if at all possible.

Try a piece of plastic like a office chair carpet protector in lieu of the plywood...(rots to fast)to spread weight out. Unfortunately this all adds up to quite a bit of weight which will only make it sink more over time I'd assume.

Maybe skip the pallets and crate/container and simply sink brush. The snow fence will work but can be tricky to keep together. Large limbs in a criss cross pattern first, then the smaller limbs and branches should form a pretty good base for future additional wood.

If interested in artificial materials, just add the larger diameter "plate" to the bottom of your buckets. A plastic fence picket cut in half would work well fastened to the bottom to spread the footprint?