Here’s a Google Sketchup design for a simple top-bar beehive.
Only 2 measurements really matter in the design of a top-bar beehive: the angle of the side panels (70 degrees) & the width of the top bars 35mm. They both pertain to bee behavior and this design has them both optimized. From what I gather, other measures are quite forgiving.
This design is simple & well researched, I do not know yet how it will fare in practice, more to come on that.
All you’ll need as far as wood is concerned is a couple of 2x12x16 and a 3/4″ sheet of plywood: