A cover (duvet) can be made out of sheets. The top of a king sized mattress is equal to 2 XL twins. I learned this when researching split king mattresses. If your duvet is too wide for your folded quilt, how about running a straight seam up one side of it to make it more narrow? Then I would do the pinning that Dina suggested and then sew at those places if it stays as you want it.