How to make a Toddler Sleep Sac without a pattern

Emma has been growing like a weed, her old sleeping bag is too short for her and a little fitted around the armholes. I had a few hours to spare today and thought it was time to make a nice warm one for this winter.

I will share the steps I used to make her new sleeping sac/bag without using a pattern...
I recommend this as a project for intermediate sewers and assumes some experience with inserting zips and attaching bias binding.

1. I choose to use a thrifted vintage woollen blanket which was going cheap due to several holes, some vintage cotton flannel (pre-washed) and the longest zip I could find in my sewing room. You will also need some bias binding for the neck and arm holes.
2. You will need to use the old sleeping bag as the pattern template.
3. With a washable marker roughly sketch the new sleeping bag shape using the old bag as a guide. I added about a 2 finger width on the body, made the armholes a bit deeper and longer and made the total length about 20cm longer. You can tweak the shape a little as you go.
4. Cutting the back: To get a symmetrical body fold the blanket in half and cut on your markings.
5. You can see how much larger the new back panel is compared to the old sleeping bag.
6. Cutting the front: On a fold place the folded back panel to use as your pattern but place it 1cm in, this will be your seam allowance to insert zip.
7. Cut front and then cut along centre fold to make the 2 halves of the front. Cut a V panel out of the front using the original bag as a guide.
8. Use your back and front blanket panels to cut the same pieces in the lining fabric. (It's a good idea to have the lining a little bit smaller than the outside but I'll tell you how to adjust this at the sewing stage)
9. Insert the zip using this technique but skip the top stitching stage for now.
10. Because the zip doesn't go all the way down, you will need to continue the seam down on the zipper line. Fold your lining onto itself (right sides together and seam, do the same for blanket front panel pieces)

11. Your seam should look like this
12. Now its time to sew the backs to the fronts. Blanket outer, right sides together, 1cm seam allowance, sew around from under arm to under arm, and then the top of the shoulders. Do the same with the lining but use a 1.5-2cm seam allowance. (this will make the lining sit in more comfortably)
12a. Make sure you leave the arm openings open!
13. Turn it all the right way, lining in and outer out.
14. So the zip doesn't get caught in the lining, top stitch around the zip.

15. Pin and trim any protruding lining.
16. Attach the bias binding around the neck and arm holes. I haven't found an adequate tutorial to explain this. Yell out if you want me to post one here.
So what do you think. I have saved myself around $80, the cost of a new sleeping bag, just by making my own with material from my stash. I think this one would be warmer and much nicer than any bought ones.
I am quite pleased with how it turned out, I have a fear of projects with zips so I pleasantly surprised myself.

I would love your feedback if you give this a try.