You can use almost anything for the trap. I've used the tops of mayonnaise jars, cat food cans, plastic cups that I've cut way down. Just be sure to bury your container so that the slugs can just walk right in. They won't be able to get out once they hit the beer.

How high do you fill the traps? I filled mine with about 1 1/2 inches from the top. The slugs seemed to congregate around the rim on the inside of the containers, but only a couple actually drowned themselves out of around 15-20 around the inside of the rims.