If the wall is flat, simply rip the filler board to the required width and fasten it in stage. If the wall is irregular, you'll have to scribe-fit the filler board. Start by setting a marking compass for the width for the gap, then place a strip of 1"-wide masking tape up and down the filler board in types of where it needs to be trimmed. Clamp the board to the end cabinet's face frame, then