simply put Batgirl, the weak spot is the fact that once it is carved/cut, it is porous inside. all those little air bubbles create little pockets for the water to escape. you can seal it with epoxy, like you said, or silicone or and equivalent. what you can try to alleviate your frustration is this: instead of a slope for your pools, step it up. make it a staircase. place a pool on a couple stairs. then you can use GS to surround them, stabilize them, and you can carve it to overflow where YOU want the water to go. plus, it will be more stable than the slope. those bins will get heavy when filled with water.