It appears that your outer borders are made up of either FG and HST or entirely HST and those are the sections you have not sewn yet. I Would do a 1/4” black border on both sides of the border. That would mean you would only have to makeup 1/2” total. I would do as Kalamazoo suggested and sew those HST seams each a few threads narrower, enough to make up the 1/2”. bTW your quilt is beautiful.