I use a Ultralife Fish Trap - 4 inch x 9 inch that I got from Foster Smith.
I put it on the outside of the tank first for a couple days, then put it in the tank. This way the fish get used to seeing it. After about 3 days, I take the cap off the one end that has a small hole and put in pellets. They go in and out of the trap to eat and think it's a feeding trough.
So, whenever I need to catch some chromis, I use this and take them out and put them in my QT tank to keep it cycled. The little goofs love to see me put that thing in the tank now
You may want to try your local reef club here in RC to see if anyone has the trap available for you to borrow.