You could also instead of sashing use a coordinating fabric and just make alternating 12" squares.
It would be sailboat , 12"sq, sailboat, 12" sq, sailboat
the next row would have the opposite blocks underneath . That Would still give you 5-12" blocks across and with seven down you will still end up with the row starting with a sailboat the same as the top row.
Then you can just add borders.
Good Luck, let me know if you have questions.
Deb