Typical machine graded common lumber (typically cheap spruce) lumber, non structurul grade has a compression stress level of 1,600 to 2,200 pounds per square inch. That means just four corner posts 1.2 inches square could support your 300 gallon tank full of water. That would also mean a cabinet 72 inches long and 30 inches wide made of 3/4" plywood could hold an aquarium of that dimension that weighed 60 times as much as your tank will full of water. General carpentry is not rocket science. A general carpenter would build a cabinet probably 100 times stronger than needed, a cabinet maker would make one 30 to 40 times stronger than needed. A premade aquarium cabinet of cheap grade is about 25 times stronger than needed, where as a good quality one is about 30 to 50 times stronger than needed. An in experienced person generally builds around 100 to 200 times stronger than needed. As long as you buy lumber without a lot of splits or knots, use water proof glue on all joints, use screws and not nails, even on your covering (trimwill cover the screw holes), and finish the inside with several coats of spar varnish it will hold for up decades. For a carpenter up here to build a 2" x 4" frame covered with nice looking plywood would run maybe $50 and a six pack of beer and a couple sandwichs. And carpenters are expensive in Alaska. Really, it is not a difficualt task.