I would square up the top and do a small solid border around it because it will make it easier to bind, given that it is your first quilt. I would use the panel for the back with sashing around it to make it a bit larger than the front. Use a nice flat-laying batting like Quilter's Dream 70/30 or Hobbs 80/20 and use spray quilter's adhesive to make your "sandwich." This will keep your backing from slipping out of place after you very carefully position it on the back (I would mark the corners of the front quilted piece with straight pins through the top and batting to show allow you to center the back panel properly as you are adhering it to the batting that has the spray on it). Then machine quilt with straight lines, square up again and do your binding. Have fun!