How do you flip a single wall of a thin extrude?

Hello! I am trying to perform a single thin extrude on multiple, disconnected sketch lines to make opposite walls of a box. Both walls need to face the center, but Onshape thinks I want one of the walls to face away from the center (see attached screenshot - the extrude on the left is connected to the rest of the part, but the extrude on the right creates a separate body). I am able to use the "flip walls" button, but it flips both walls at once, so one wall is always in the wrong place. Is there a way to flip individual walls so I can put them where I want without having to do separate extrudes? Thanks!