I would think that for doors like this, you want the horizontal pieces
to determine the width of the door, with the wide vertical panels more
"floating". Use wide rabbets on the edge of each panel-board so the
boards overlap, and if they expand or contract it doesn't cause gaps.
One fastener per board, near the edge that's on "top" rabbet-wise, so
it holds two boards down without restricting expansion.

You'll need a diagonal cross-brace to keep the door from sagging, of
course.

Option B is to rabbet the column an inch or so across, so the door
overlaps the post more but won't stick if it expands a lot.