Since a queen is approximately 90x96" you will need about 24 inch of borders all the way around.
What fabric do you have to use?
You could do a 6" border, then a 12" block and another 6" border or any combination to make it 24".
Could you continue the blocks you have with that same fabric? That would look nice for another 18" then a 6" border.
So many possibilities.
You might want to consider getting another of the same panel and use some of that.