If you're trying to save money for a larger tank, I'd hold off on getting/building a refugium for your existing one (unless you're a couple years away from that new one).
That being said.... The design idea is fairly simple. Consider a sump... is really nothing more than a container for some water, w/ hopefully a few baffles in it, and the water flows from one end (where it is received from the tank), to the other (where it is pumped back to the tank).
Following me so far?
Somewhere in between those two area's, you want a couple baffles, and there's your refugium!
| Input | refuge | Output |
|______|_________|__________|
That's really all there is to it. You can get crazy, and do all sorts of things, but that's a little more involved, and like I said, if I was moving to a bigger tank, I wouldn 't waste any more money upgrading my existing one.
I speak from experience there as well (not to change the topic or anything, but...) Built a new hood for my 90-reef, and when it was almost finished, the Mrs. told me I could get a bigger tank! Know anyone that wants a never-used, brand new DIY hood for a 90 gallon?
- Mac