I've never used a 2 liter to make a trap but i have used traps before and had a lot of success with them. It just takes a little patience. My LFS
has a fish trap that you could rent for a couple dollars a day. I would ask around and see if any of your LFS
will offer the same.
When you set the trap put some food in the back and let the fish get comfortable with the trap being in the tank for about 2 hours. You don't have to watch to closely because they most likely will not go in yet. Then get a little more food and make sure you make a trail from the back of the trap to the opening and a little bit just outside the trap. I find the fish are just not quite smart enough to figure out how to get in the traps unless you lead them into it will a trail of bread crumbs.