T.J. Carr and John Bradford have a beautifully engineered plan for a top bar hive. You can find the newly updated plans, released October of 2012 here:

Plans for a top bar honey straining rack, designed by T.J. CarrHoney Straining Rack

Les Crowder’s simple and practical Topbar Hive plans can be found here: Crowder Hive Plans

Other links to topbar plans