if it were my tank i would try to go considerably wider then 12" (depends on the size of the tank but 12" wont provide very good surface skimming on a tank any bigger then 10-20 gallon.
you dont want the cut to be square in the corners, you want the corners to have a large radius to prevent it from cracking...
i never said it wouldnt work, just that a longer overflow will skim the surface of the water better as it wont have the weir height that a shorter overflow has, essentially skimming just the surface of the display and not the top 1/2"
BTW your tank is beautiful