I’ve got it now! Basically, you just need to add a lone stile up agains the wall, or do you want a plywood “side” attached to it? Either way, CabWriter doesn’t do that automatically, but you can easily draw those parts using SketchUp commands, then turn them into CabWriter recognized parts using the “Make CabWriter Component” command. Alternatively, you could duplicate other parts in your drawing, like a stile with an attached side, or just the stile and move it into place and modify it as needed, then use the “Make CabWriter Component” command to change it into a unique part.
Hope this helps!