Good Day MylesG,
Thanks for your post.
I would recommend to use the Wall Surface Tool. When doing this select a plasterboard material for the 'Surface' field (note: you will probably need to create a material) then select 'None' for the other fields. If you were to manually edit the geometry it may work but unless it's done correctly it may not appear in the takeoff, plus manual edits wouldn't be remembered if the PlusSpec object is edited/redrawn at all using one of the PlusSpec tools.
Could you please try this and let me know how it goes?