Regarding the Fasttech Subtank clone parts that you can buy and put together a top-filling subtank for about $12:
I love them and they're the only tank setup that I'm using these days, BUT...
After you refill them from the top, they ARE going to leak, and leak quite a bit, for about the first 10 draws before the leaking stops. I don't know the reason but if I had to guess, I'd say it might have something to do with vacuum having to get re-established after filling. You can, if you want, fill them from the bottom just like you would with a Kanger Subtank, but to me the leaking is something I'm willing to deal with in exchange for the convenience of top-filling.