Hi all.
When I started thinking about this pattern I had two objectives: first, they must sink slowly, as much as I could, like a real one. Second, they must sink in the right position, with its legs downwards. You can see at the next pic how it rests at the bottom of a glass, full of water. This is also important as you can find shrimps at still waters walking over stones or mud.
I used foam, rubber, two ostrich feathers, "Realistic Shrimp Shield", and foam (instead of lead wire). Foam helps me to bring out those two objectives.