I must say I like Veriann's idear but you can also do one of the hang on back styles that a few companies offer. I've been using a 2.5 gallon hang on back
fuge on a smaller tank for a while now with great success.
Bear in mind lighting constraints while your designing as well, if you plan to run
macro algae in the
refugium your going to want to light it somehow. I've had great luck with the little 25 watt halogen under cabinet "pucks" from home depot (had for pretty cheap, great spectrum, and teeney!) which may buy you space if that's a problem but at the cost of a little extra heat put off the bulb vs. flour. lighting. Do let us know what you do though!