Since you have a longarm, I might pre quilt a heavy backing fabric with all your leftover batting in a simple 4 inch grid pattern. I would then sew a simple or do a whole cloth top. I would put the top with a polyester batt under it underneath of prepared backing and use the long arm to tack the layers together following the 4 inch grid pattern already on the backing. You could tie tack yarn at the 4 inch corners and then tie the ends after for a pretend yarn tie effect.