Darren, you wont want to use finger or box joints for your boxes. The reason is you would be exposing the plywood laminations on the fingers to the weather and your box would come apart in no time flat. Instead consider using a corner block of solid wood that is both glued and screwed in place. You will also want to wrap the exposed edges in solid wood or at the very least apply a veneer of solid wood to help seal the plywood edges from the weather.(A much less expensive method) For your finish you will need to use a good spar varnish with UV inhibitors. Plywood laminations do not react kindly to outside weather, and anything you can do to protect them from the sun and water will increase the life span. If you have questions about different ways to attach corner blocks I will be happy to give you some ideas.