Many manufactured (and DIY) aquarium cabinets do not use a solid piece of wood under the aquarium. Instead, (to save material and money) they use wood strips around the perimeter and a front to back strip in the center of the cabinet.
Oak plywood and trim are expensive. Even with the 2 sheets of plywood suggested for this plan, there isn't much room for errors. Making practice cuts on scrap wood when you change blade angles and depth of cuts makes a lot of sense.
The materials used for both the cabinet and matching canopy were purchased at Lowes (with the exception of the hinges) and cost about $150. The hinges were purchased online and cost about $16 (including shipping).