Put on your safety goggles and use your circular saw to cut a length of MDF (Medium-Density Fiberboard) to the length that you would like for the bar top. MDF board can be purchased at any hardware store and is inexpensive.
Use the circular saw to cut the MDF board so that the width matches that of the sides of the bar you intend to place it on.
Place the bar top over the bar and use your power drill to drill the ends of the bar top into the side frame pieces.
Apply a thin coat of varnish to the surface of the bar top and allow it to dry.