I just made one of these, but here's tip if you cut the outer ring off of a bio ball it will fit nicely in the 1.25" pvc blocking any carbon from escaping into the tank. just place one on either end with the carbon in the middle. No polyfloss to trap crud and no carbon coming out of the end. Good luck!
I've noticed an improvement in water clarity since I ditched the mesh bag and put the reactor online. It's also a LOT easier to change the carbon when it's in a reactor vs a mesh bag.
Moe, from my experience a ball valve will do the same job as a gate valve. Only drawback is they're harder to adjust and trap more nasties. But they still work. I have a few ball valves running restricted flow applications on my system successfully.