I forgot to ask one other question: is your tank pre-drilled for an internal overflow? My guess is no, but I wanted to make sure.
For under the tank I would just use one or more smaller tanks or rubbermaid bins (or one each). Under one basic system, you would T the overflow line and drain most tank water to the "sump" and a smaller amount to a "refugium." The refugium could be a separate bin or simply a baffled section of the sump. In either case, have the refugium gravity drain to the main sump, and return to the display from there.
The other basic system is using a single bin and single drain line, but simply baffling some section of the sump off as a place to put rock and/or sand. With small sumps, I'd probably recomment the first system above, so you don't get too much flow through the refugium.
Tim