I tried the bottle trap, I thought it was useless. A trap I improvised that worked well once I ironed out the kinks is as follows...
Take a piece of PVC tubing, cover one end in Saran wrap and hold it in place with an elastic band. Put a little trail of food inside with most at the closed off end. When the damsel goes in for the food simply stand the tube so the open end is facing up (don't panic, fish don't know they can swim up to freedom, they think they're stuck)..VERY IMPORTANT..hold the closed end when you lift it out or else the water pressure will blow it open and you start all over again. Worked for me, it's cheap and easy and you don't have to chase the fish around the tank or break down your rocks. Good luck.