I was intending to leave it open but I guess a sock could be put over the end.
As a permeable barrier I have been using shade cloth, weed mat and window screen, pretty much anything I have lying around really. I fully expect to have to change it at some stage as none of those items were meant to be kept permanently wet for long periods of time, others have told me they use vermiculte, or
straw as a barrier. The weed mat may last a bit longer but I only used it because it was permeable, not to keep the weeds out, it isn't the woven plastic type of weed mat, just the felty material type.
I only just found out about them myself recently, I am not sure how long the modern ones have been running for but I would assume that the earth would need replenishing or the roots would need to be dug out eventually.