Thanks for the suggestions as to how I could add prairie points to my aqua Christmas points, I think I have found the way to do it.
The points are pinned in place and the binding added to hold them down. I used 4 pieces of binding, one for each side, but rather than overlap them at all, each one finishes on a 45o fold at the edge of the fabric
Trim away as much as you dare from the corner
Flip the whole of the binding, plus the seam allowance, over to the back, and pin, ready to slip stitch. I reckon when I've stitched the mitres to each other and the binding to the back it'll be sorted!
I has just two squares left, and a few triangles, so under my "use it all up" policy they needed to be used, so I adapted Svetlana's draw string bag by adding another fabric to the bag parts, and the triangles between