Without seeing the project.. this is how I think I'd deal with it. I would piece batting in and add a strip by machine to one side of the area, make the strip large enough to cover and turn under. Sew it onto the seam allowance and then fold over to cover batting, the other side should be pressed to cover the other side. Then hand stitch it down. You may have some areas where you have to hand stitch the ends too because of the odd shaped blocks. I made a quilt years ago that was like that and it was the only way I could figure out how to finish the back. It worked out ok. The other option would be to put your batting in and cover the whole back with one piece of fabric and re-quilt stitch in the ditch around each block. It would give a more cohesive look to the back and in the long run be easier, but more costly.