I am reminded of "Flatland" where there is a pronounced horizontal gravitation which gets stronger and stronger depending on the region. Since someone on the edge of the world would have the entire earth to one side, they would feel a pull in that direction; someone in the center would only be pulled downward. You could decide just how strong this pull is, from making it impossible to reach the edge to being just a small but noticeable force. This would be easy to implement and a fairly clear hint as to the shape of the world, as opposed to any bizarre astronomic effects.