In addition to the wonderful suggestions above, I always think you can't go wrong with a 2" grid crosshatch done on point. You can mark it off with 2" Painters tape and quilt right next to the tape. When you get to the corner simply turn around the quilt and stitch back up the other side of the tape. Peel off the tape and place it going to the opposite corners. Quilt on both sides. You now have a big X quilted which will stabilize the whole sandwich nicely. Continue placing tape and quilting alternating directions as you work your way out. You should be able to peel and restick the tape several times (4ish) before you have to peel off a new piece of tape. It is quick and easy.