If you have the skill to do a good job on the cab, then the only advantage to buying a factory box is that you know ahead of time what it sounds like. If you look at this thread and follow the link in my post there, you should have enough basic info to get started on your design. For a bass cab, you need to make it very rigid, so you should allow for some heavy bracing on each panel. You might want to use 1 inch plywood instead of 3/4 inch. Good luck!