Sorry, cut a piece 18 inches long and then cut it diagonally from corner to corner, essentially turing your rectangle into two traingles. This will give you two slanted braces to lay your top on. Repeat the same thing for the second side.
Did that help any?
Edited: If you want the front 2 inches off the ground and the back 4 inches, this obviously won't do it for you. This will lay the front of the board pretty much flat on the ground and back will be 6 inches off the ground.
If you go this route you will end up having about 3 inches of overhang in the front and the back and then I would space them so that you also end up with 3 inches overhang on the left and right side.
[This message has been edited by 03_Aggie (edited 4/23/2010 10:03a).]