I have a 28 1/2 square septic top to cover that is 12 inches high.  This item would not fit over the top because of the variant in sizes in the top, so it does not fit a 12 inch high top.  I had to make it fit over the top, but it never fit flush with the ground.  This is the second order, and neither of them had the inside measurements that worked for my septic top.