It's a lovely quilt!!
My inclination is that you do not need a border! Just bind it with a scrappy binding of the fabrics you have used.
Do you need the extra 6"? or could you accomplish that with a bed skit?
If you need the width ... just go with a plain white border, no other fabrics. Do some quilting in that white space. And do the scrappy binding.
The plain border will not take away from the quilt, whereas print blocks would, as you're going into something different.