I was trying to figure out how to do the same thing with my GFG quilt when I get to that stage. Been thinking about it and here is what I think may work for me: I'm going to layer the quilt as far to the edges as I can and keep a straight line then quilt, then I am going to take another flower that was pieced and lay it over the edge one on top of the batting and backing and whip stitch together outer edges and hand sew down the edge covering the backing. I think if I go along the edges around the quilt it will kinda frame the back and keep the hexagon shape on sides of quilt. Hope this makes sense. It's still in the thought stage as I am still piecing hunderds of hexagons. Anyway, maybe this will work for your blocks.