Well, you could try a large puffer fish...
Seriously, though, there are fish traps that you can buy. If your LFS doesn't carry them (a bit out of the ordinary hardware list) they are definitely available from the large online retailers (such as Marine Depot or Dr. Foster and Smith from the RC sponsors page).
If this is a one-time experience that you don't plan to repeat often, it might make more sense to make one. There's a great thread stickied at the top of the All Things Salty forum regarding catching of fish and other critters.