Ill put it to the test with a 2L. I think putting it horizontal would be better so probably tie some weights on the ends. Enlarge the mouth for better selectionSure it isn't hard:
Take a empty soda or water bottle and cut off the top, stick a stone or two in the bigger portion of the bottle to weigh it down, now stick the top half (upside down ) in the other half, this creates a funnel that fishcan enter but have trouble getting out of