So here's a peek at attempt 2. It actually doesn't leak when I turn it upside down and shake it vigorously.
Although, since I actually ordered a Hydor Prime 10, I think I may turn this into an aquatic vacuum cleaner. Something with a decent little hose (covered with a screen to keep from vacuuming aqua soil), some mechanical filtering and a return so that a good cleaning of the tank doesn't necessarily mean a water change.