Your misjudge on the block size, would not change a rectangle quilt into a square quilt.
Instead, it would appear that you did not do all the blocks/rows that are needed.
You could add one more row to each side for a total of 10", then you just need 4" in borders.
And for the ends, you could add 4 more rows, for 20" or ... 6 more rows for 30".
Looking at the pattern, these additions would not take from the pattern and would work fine.