If you have any scraps from your bargello, you can make it work. Cut your fabric into 2, 2 yard lengths. If you sew them together side by side, you'll have 72" X 84" . You should be able to make either one 72" by 12" or 2, 72"by 6" strips to add to the length. (This is with the seams going side to side on the back instead of top to bottom.
Either add the strip through the middle, or a 6" strip at the top and bottom.
Or just use a coordinating fabric from your stash - you'd only need a yard to make a 72" by 12" strip.
Once you have the 2 back pieces and the strip(s) together, you'll have 72" by 96" (minus seam allowance) which is plenty big for a 66"by 90" quilt.
Let us know how you make out! It's a gorgeous bargello!~!