If you have anymore of the colored squares, why not make a neutral sashing of the same width, then conbined all your leftover colored squares for another sashing (I know that's tedious) and then add a 1.5 or 2 inch border of the neutral color last. Uses up any leftovers and will add the width and length you want. Maybe a bit more length than you mentioned though. Another thought if you don't have as many of the colored blocks leftover, alternate them with the neutral color.......every other one and so on.