My friend got a black wire "minnow trap" from a tackle shop for only like $15. Its completely open on one end, he removed the hinged "door". On the other end, it has a conical opening that I guess the minnows are supposed to swim into.

Don't know how else to explain it, but look for something like a minnow trap, because the holes are just small enough to keep the bigger sand crabs inside when you sift the sand out. The smallest sand crabs fall right out so that you don't have to worry about them. My $0.02.