Tinned fish

[Hrusai]

personally i'd get some foam and test it on its compression, then cut a circle of the stuff the size of the inside of the pipe, and make it so that when the whole pipe is full, the foam is compressed to the bottom, may have to experiment with the foam a bit, but its inexpensive and easy enough and its simple!

but yeah theres many avenues you can take this idea, the magnet one sounds pretty decent

and the springs a good idea but would need more experimenting/ calculations, not to mention deformation of the metal over time and that'll be more expensive to get a new spring than it would be to get some more string or foam