Freshwater sumps work pretty much the same way that a saltwater sump does, just without a protein skimmer. Google "Melev's Reef". Melev goes into great detail about sump design. With a little creativity, you can adapt his concepts to a Rubbermaid bin. An old aquarium makes a great sump too.
If you're going to inject CO2, a sump is a poor choice. It'll offgas all your CO2 when the water splashes. A canister filter is a better choice for CO2.