the way that you have it drawn from the top looks right. if the front "sides" are 13" each the driver baffle will be around 18.4" so that should be fine for mounting the driver.
looking down from the top, the dimensions are 26" square - the diagonal for the driver baffle - the slot port - the panel widths all around.
if using 3/4" thick wood, that gets you to a 24.5" square inner dimension, which is about 600 square inches.
then take 7/8 of that for the corner that is chopped off where the baffle will mount, so that leaves about 525 square inches.
then we have to remove the port area, which as you have drawn it is about 21.25" x 3.75", so that is another 80 square inches.
that leaves the net area of the inside base at roughly 445 square inches, or about 3.1 square feet.