Purchase a container of these exterior screws.
You’ll line up your boards.
I used 3 boards, side by side. face down.
Attach these boards using the smaller board you had cut, the batten (placed UNDER the boards while your drilling).
Use a piece of cardboard as a template to line up where the batten will sit, so it will be equal on all shutters.
Drilling from the back of the shutter eliminates the need to counter sink and fill with wood fill.
Do the same thing on the other end of the shutter, and…ta da.
You just made your own shutter!
now, repeat 11 more times.