This cutlist should work but be sure to double check all my math. I'd hate to have you mess something up because I added wrong and I am not always the best with numbers.

You need:

7 pieces 24 1/2 X 17 1/2 or 24.5 X 17.5 inches in both cases

2 side pieces 24 1/2 X 40 1/4 or 24.5 X 40.25 inches

and if you want a back

1 back piece 17 1/2 X 40 1/4 or 17.5 X 40.25

You'll need two sheets