This cannot be done with the round board template.
Start with a square board outline with the sides equal to the diameter of your round board. I recommend centering the board outline around the origin. Mouse over a corner of the board outline, right click and select convert corner to arc a. Enter in the radius of your circle to create the arc. Repeat these steps with each remaining corner.
To create the flat side: Mouse over the side you want to flatten and press the 'd' key on your keyboard. This will delete corners and will allow you to flatten the outline. You may want to place a temporary line designating where you want the flat portion to exist to use a reference point so you don't remove to much of the rounded outline.