Yeah, those skimmers are really made to drain into something... You cannot seal it or it kills the skimmer, in fact that is how the ETSS waste collectors work, they use a ping pong ball that seals the air exit when the cup is full.. Basically stops the skimmer, but keeps it from overflowing skimmate too... Drain the hose into a mason jar, or similar if you dont want to spring for their waste containers. Make sure whatever it is lets enough air out unobstructed for best performance. If you drop the waste hose a little bit into the container, when skimmate gets high enough it will slow down the skimmer, though not completely shut it off.