I love these reactors. I had a chain of three on my reef (Seachem Matrix Carbon in one, then GFO in the next, and Purigen in the last). Wrapping 'the bag' around the sponge that comes in it or using some sort of fine mesh/pad should be more than adequate to keep it contained.
I can definitely see how used out of the box they could leak, but I just used some of their plastic hose clips on all the tubing connections and have never had any issues. I personally have a hard time justifying the big jump in price between the Phosban Reactor and other reactors out there given they are not much more than plastic chamber.