a 5.5 gallon would be a perfect refugium for a 10 gallon. plans will differ from person to person but the main thing you want to focus on is flow of water through your refugium. u dont want to have the thing only to have the top layers of water flowing with the lower portions stagnant. if you can think of a good way to do it i would suggest having the refugium above the 10 gallon tank so any pods and such that are beneficial to the system will free fall into the show system without being destroyed by a pump like many underneath refugiums.

talk with malofish about making an above if thats what you want, he figured out a great one that worked just fine.

hope it helps, steve

ps dont forget to completely rinse out the tank and clean it really well, it shouldnt matter too too much but just to be on the safe side, u might even want to rinse it a bit with some vinegar.