Generally, if a quilt is going to hang, I want the top & bottom to be 1 solid strip to support the quilt.
With your quilt, I think a corner square would look great, especially if you use 1 of the prints. I would add the sides first, piece the top & bottom with a square, & add them to the quilt, matching the seam.
It follows suit that you have squares in the sashing as well.
Great job, the sashing really accents the quilt beautifully.