I had a 8 gallon bio-ball type sump on my 40 gallon. I modified it by adding a third wall to create the over under over (also made it 1 inch apart so I could run charcoal filters when algae got bad) I added live rock where the bio-balls were and floated chaeto over the live rock. (didn't have a skimmer chamber) Worked great and had lotsa pods until I put some Peppermint Shrimp in there, they ate my pods. (big no-no and I had to break it down to get them out) I just modified a old PC light from an Eclipse, worked great.
melevsreef.com has some great info on sumps and refuges