If you have a plastic bottle and a cutter/knife/scissors, that's great --- you can make a fish trap in a minute -- it is so satisfying to catch a fish!
That is the reason I make this Instructable.
Features of this design:
Rocks in the bottle --- sinks the bottle; stabilize the bottle against the waves.
Small bottle --- easier to lift when filled with water; will be harder for small fish to escape through this small opening than the 5L bottle opening.
Cut the bottle at a low level --- the top and the bottom part will fix together firmly without tape, glue, fishing line, or wire.
Not to use fishing line --- easier to opening/close the trap; easier to make the trap.