30 seconds to do, forever to describe.......
Basically, take a 2 liter soda bottle, cut out the neck of the bottle a little larger than the fish, then drop down several inches beyond/below the tapered shoulder of the bottle and cut the bottle in half..... Then (this is the part where the written word fails people in this description), take the smaller piece (the neck openning) and invert it (flip it over) into the open end of the bottom of the container. Tape might be needed to hold it in place as surface friction may not be enough depending on how straight and clean your cuts are.
Clear as mud?