My absolute favorite method to "fill in" the back is to make extra blocks or use the blocks that aren't quite good enough for the back and run a strip of blocks down the back. (or across the back if it is better). I make 48 x 72 throws so, I only need about a block's width to add. If I don't have extra blocks, then I sew squares of the fabric from the front and use that. Of course, squares you could go 2, 3, 4 or more to add width.