I think buy better lah... 6x4x3 is like 2 tons of water. Unless you are very sure that you can build one to withstand that kind of weight and pressure, buying is better peace of mind.
If you were to construct a steel frame to fit the plywood tank in then it would easily be able to withstand 2 tons of pressure but it seems like to much trouble to go threw to make such a tank.
Has been done many times before, the basics are you build a reinforced plywood box, leave a window in at least one side, line the box with fibreglass and 2 part resin, place either glass or acrylic sheet in the window and there you have it! Of course there is alot more to it than that but thats the very basics. When my Murray Cod gets too big I hope to look at building one myself.
Heres some links to some other peoples plywood tanks: