Do you have any of the borders on? To make it 90" you need to add 5" on each side. I would make several borders 1-2" (finished) using the same colors that are in the top. So that means original borders from pattern and then 2-3 thinner borders. Audition different colors until you find one that you like. If you have any of the hour glass fabric left you could make cornerstones from it or make corners stones using the churndash pattern. By doing corners stones you can sew the borders together and attach as one piece.