I agree with Kerik - cut the mat's to keep the proportions to the printed panoramic image.

5x12 = 11x18
7x17 = 14x24
8x20 = 16x28
12x20 = 20x28

As the prints move up and down in size I also feel the width of the border should adjust accordingly... you can see that from 5x12 (~3") to 7x17 (~3.5") to 8x20/12x20 (~4").

Hope this helps!

Steve N