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I personally do not have a sump but many of the mods here can point you in the right direction. Maybe steve-s or hara can chime in. You can try using the search feature of the forum. These types of things have been gone over several times.
A 20 gallon long (if you can fit it under the tank, most stands won't let you put a 10 underneath without mods done to it) would give you more than enough room to utilize it as a sump/refug... I have a 10 on my 55 and wish I had more... :x