Yes very unseemly to see seams...
Sometimes it takes a little more care and work to make something look as simple as possible. Seams are a visual complication.
I try to buy my matboard 25 32x40 sheets at a time. I feel secure knowing I have almost unlimited matboard. Out of the 32x40 I cut one 24x28 (for 16x20 prints), one 16x20 (for 8x10 prints) and one 12x16 (for 5x7 and 4x10 prints). That leaves me with a 4x24 extra piece.
The hole I cut for my 16x20 prints out of the 24x28 is actually slightly bigger than that, so the hole becomes another 16x20. The hole for the 8x10 prints are also a little larger, so I get an extra 8x10.
So out of a 32x40 I get:
Fairly efficient usage.