Right, here is my basic idea (with cost saving in mind!
Basically, use the two large side peices to give you the structure of the unit fixed in place to the base piece and a frame top (with screen stapled to it - or joined however you want). This will give you the basic shape and give the structure all the rigidity it will need.
Then screw on small front and back pieces to make a base like mine which you can fill with a plastic membrane etc to make the planter.
The back will just be the screen stapled in place into the side pieces with the front being a hinged sceen door.
It won't need many materials but i would stress that you will nee to seal the wood on the wide well (i'd use yacht varnish personally - with plenty of time to air - but there are lots of alternatives)
Let me know your thoughts!