I see Holice's point, but wonder about the non-square size. It seems to 'block' the challenge (pun intended) if a single block was the intent.
How about taking a four patch block that is made up of only HSTs, like Dutchman's Puzzle, and making a 6" log cabin block to use in place of each HST. You'd wind up with one block (the Dutchman's Puzzle) made from 16 other 'one' blocks (the Log Cabins). That would cover both possible interpretations of the challenge...or would it just be a two block quilt that way? And you'd still have to fudge the extra six inches.