Well, I'm racking my brain right now...
I can get one side to seal properly, but the other is leaking very slowly.
I'm still working on it though! I've glued the male side of the couplers into the box to eliminate one possible leak point. Somehow, I still get a slight leak on the other side! I have a gasket between the two boxes, and a gasket on the inside of the female side. Still, it drips! Only on one of the two spillways, though.
I've added a little plumber's grease to the gaskets to try and generate a better seal, and it looks to have aided slightly. As soon as I can come up with a feasable solution, I'll post my results.
I am NOT defeated yet!!