I built mine almost exactly like that although the dimensions werent the same, I used 2 braces in the 90 degree corner, and my leading corner brace is positioned like your top braces. When you go to screw down your surface make sure its plenty long, start screwing on the bottom, and cut the extra off. I redid my top twice because the first time it was too short. These are pretty horrible pics but you get the idea.

Edit: To make semi nice drawings try downloading google sketchup. You can make 3d models, it free, and its surprisingly easy to use.